This paper introduces an economic model to explain the relationship between traditional telephony services and VOIP services. In particular, we show that low income families have not benefited from VOIP services and VOIP could be a substitute for long-distance calls but complementary to local services.
Trang 11 Introduction
The bundling game between
Telecommunica-tion companies (Telcos) and Cable TV is creating
a new convergent trend called triple play
includ-ing TV, broadband network and telephony This
is leading to a new dynamic competitive
environ-ment in digital communications The traditional
service providers are now playing two main roles
as incumbents as well as rivals In other words,
the giant incumbents in telecom industry are
striving to increase the cable television market
share where cable TV companies have dominated
the market for a long time Meanwhile, cable TV
companies have also developed new technologies
to implement bundling triple play services and
be-come stronger competitors in telecom services
Otherwise, by using the available advanced
tech-nologies based on the global network, many new
service providers could enter the market and
com-pete with the incumbents in traditional services
For instance, on the one hand, VOIP providers are
offering a bundling of voice services with a
com-parative price or free of charge in some cases such
as Vonage, Skype, Guzimo, Yahoo voice, Google
voice and so forth On the other hand, internet
protocol television (IPTV) providers and Telcos
are soaring to capture the market share of the
dig-ital video on demand, telephones and television Therefore, the telecom and cable TV markets have become more dynamic, complicated and competi-tive in the past few years
By building an economic model, the paper an-alyzes the relationship between traditional teleph-ony services and VOIP services We will provide some evidences to prove that (1) low-income households have not widely used VOIP services even though VOIP telephone is supposed to be cheaper than traditional telephony; and (2) VOIP services could be a substitute for long-distance calls but complementary to local services Specifi-cally, by collecting data from all states of the USA
in the period 1995-2006, we implement an empir-ical analysis of bundling telecommunications serv-ices to test our hypotheses We also provide a summary of VOIP services in Vietnam and com-pare them with the services operating in the US This paper is organized into six sections Sec-tion 1 provides the introducSec-tion, while SecSec-tion 2 presents a review of some main prior studies of telecommunication market Theoretical model is presented in Section 3 Section 4 implements an empirical study The VOIP services in Vietnam are presented in Section 5 Finally, Section 6 sum-marizes the study’s results and assert some
limi-Voice Over Internet Protocol (VOIP) providers is generating the game of telecommu-nication market Based on the available advanced technologies and smart soft-wares, new entrants can reduce initial investment and production costs to compete with incum-bent local exchange carriers (ILECs), which dominate the telecommunication market for
a long time due to economy of scale and scope This paper introduces an economic model
to explain the relationship between traditional telephony services and VOIP services In particular, we show that (1) low income families have not benefited from VOIP services; and (2) VOIP could be a substitute for long-distance calls but complementary to local services Specifically, we implement an empirical analysis of bundling telecommunica-tions services that are carried out with pooled cross sectional time-series data for all states of the United States of America in the period 1995-2006 Finally, we offer a brief overview on current situation of VOIP services in Vietnam.
Keywords: competition, complement, substitution, telecom, VOIP.
Trang 2tations that are challenges for future research.
2 Literature review
In general, consumers would like to maximize
their utilities under a budget constraint However,
in the internet telephony market, the prices are
continuing to decrease, hence the current prices of
telephone services are more likely to be under
their expected expenditure It is not easy to
per-suade them to switch from an existing provider to
a new one in a short time For instance, many
con-sumers are not ready to switch from fixed line
services to VOIP services The replacement of
tra-ditional telephony with the new technological
ap-plicability such as VOIP is not easy For instance,
Fowler (2002) analyzes the simplification of User’s
life: New technologies are a tough sell unless users
can perceive tangible benefits Consumers do not
respond to “technological elegance”, but to
con-crete improvements and documentable savings
Replacing something such as the Plain Old
Tele-phone Service (POTS) is a difficult challenge
be-cause it is familiar, exceedingly reliable, easy to
use, and matching users’ requirements
In the telecom market, network access and
net-work interconnection are main issues for a new
entrant to enter this market These problems are
considered as a major barrier to the telecom
com-petition In reality, ILECs usually find many
rea-sons to postpone new entrants to connect to their
network, especially in countries where the telecom
market is still a monopoly or an oligopoly
Vogel-sang (2003) analyzes the differences between an
access model and an interconnection model, in
other words, one-way access and two-way access
Loomis and Swam (2005) investigate the telecom
competition between ILECs and CLECs in the US
They find significant inter-modal competitive
im-pacts resulting from wireless and high speed
de-velopment
The rapid emergence of VOIP is considered as
a real threat to traditional fixed line telephone
since VOIP providers enable to reduce production
cost by absorbing technological novelty to create
advanced services However, internet telephony
also brings negative effects on a creative
destruc-tion Murillo and McKnight (2005) study the
im-pact of internet telephony on the Universal
Service program in the US They find out many
causes leading the process of creative destruction
They are debating in the US whether VOIP is
an information service or telephone application
This issue becomes more complicated because if it
is considered as a telecom service, then it should
be subjected to any or all of the various common carrier regulations In this case, ILECs can easily dominate the telecom market driving new VOIP entries out of the market (Shin, 2006)
The major issue of VOIP services is quality
However, this problem is able to overcome due to technological innovations and intelligent soft-wares Internet telephony is changing in tradi-tional communication cultures For instance, Skype is considered as one of prior businesses in VOIP services Tapio (2005) analyzes the success
of Skype in both technological solution and busi-nesswise Additionally, VOIP services are requir-ing the new regulatory framework Goncalves (2005) studies this issue and uses Skype as a case study Skype has proven to be increasingly popu-lar among business users because of low cost, less time, simplicity, and productivity improvement
Skype has continued its meteoric rise in interna-tional call market share, which now has 25% of the international calling market and is serving up some 520 million minutes of calls a day (John Nor-ris, RCR Wireless, Jan 6, 2011) Specifically, in January 2011, after the release of video calling on the Skype users for iPhone, Skype reached a record 27 million simultaneous online users (Vlad Savov, www.engadget.com, Jan 11, 2011)
The VOIP progress is conditioned by the regu-latory framework designed for it Feijóo et al (2007) analyze 5 main VOIP development scenar-ios that represent an adjustment of the specific regulatory intervention in basic issues such as in-terconnection, numbering, operator obligations and user rights As a result, VOIP could achieve greater market shares and generally replace tra-ditional telephony whenever both technical and regulatory issues have been already solved
3 Model
In this part, we mainly focus on the impact of VOIP services on the telecommunication industry
In order to simplify our model, we assume that there are only 2 providers of telephone services
The existing one called Firm 1 has a monopoly power in supplying local networks (for example Verizon) Another new one called Firm 2 (for
Trang 3in-stance Vonage) enters the telephone market by
using VOIP technology to provide telephone
serv-ices with lower prserv-ices to compete with the former
one
We consider consumer behavior corresponding
to available telephone services including
tradi-tional telephone and VOIP calls If they switch to
the new provider (VOIP calls), they have to face
substantial costs of switching The switching costs
include an additional cost of using electricity for
service, learning costs and initial investments
such as adapters, special phones and connection
fee as well as service quality Obviously, the
switching costs make consumers more dependent
on the traditional phone services Another
impor-tant technical issue, VOIP providers can only
pro-vide the VOIP services for subscribers who have
already used a broadband internet access In other
words, the normal fixed line telephone or cell
phones are not completely substituted by internet
phones
We consider how much subscribers have to pay
monthly for using the traditional phones A user
usually pays fixed fee for using the local line (Fll)
and pro base rate for using long-distance calls
which are calculated by summation of all price per
minute (depending on the distance between
sender and receiver) multiplied with total minutes
of calling Hence, the total monthly expenditure of
long-distance calls is equal to Because the
payment for using long-distance calls is variable,
a subscriber has his or her reservation value of
using long-distance calls (R) She/he is not willing
to pay higher for the reservation value Subjecting
to the budget constraint for long-distance calls,
the expenditure on such a kind of service is more
sensitive in price Therefore, if consumers have
high demand and low reservation value of
long-distance calls, then they will have higher
poten-tial to switch to VOIP services, which are usually
fixed in price for unlimited long-distance calls
(FVOIP) However, to come with a final decision
in shifting services, it requires consumers to
con-sider many elements such as service quality,
prices, conveniences, compatible with consumers’
existing terminal equipments and so on One of
the most important factors is a switching cost (S)
It is playing an essential role in transferring to
the VOIP telephone or staying with the
tradi-tional one
We consider subscribers’ decision to switch to the new provider or staying with the existing one
as follows If consumers’ total expenditure on long-distance calls is not greater than the reservation value, then they will stay with the current provider ( ≤ R) In this case, consumers are satisfying with the traditional services even though they have to pay higher monthly prices
If the reservation value is smaller than the total expected demand for long-distance calls, con-sumers will consider the switching cost (i.e > R) In this case, there are no shifting services if the following condition is satisfied (R - PVOIP –
S < =0) Otherwise, consumers will transfer to the new entrant (R- PVOIP – S > 0)
Therefore, the new entrant could only provide the internet phone for consumers who have low reservation value but high expected demand for long-distance calls In other words, consumers have to meet two conditions before deciding to switch to the new entrant Furthermore, the reservation value of each consumer has strictly de-pended on her or his income According to data collection from Federal Communications Commis-sion (FCC-2005), telecommunication services ac-count for about 2% of total households’ expenditure during period between 1981 and 2003 It means that the reservation value of each consumer has been stable during the long period The higher-in-come households have greater reservation value and vice versa
Hence, the low-income families would not only like to use cheaper services than the high-income ones but also have high demand for new services However, the fact that whether this demand is satisfied or not will be a question
Proposition 1: The low-income households could be less potential for switching from a traditional phone to a VOIP phone.
The reservation value of low-income families is smaller than that of high-income ones Therefore,
if low-income households have high demand for long-distance calls, then the first necessary con-dition is satisfied ( > R) However, the second necessary condition for switching services may be not met (R - PVOIP – S > 0) The second condition
is the main factor making low-income families less potential to use the cheaper phone services In re-ality, according to the Reference Book of Rates, Price Indices, and Household Expenditures for
Trang 4Telephone Service published by FCC-2006,
low-in-come households have received the subsidy
pro-grams for using wire-line telephone services
These programs are called the Lifeline and
Up programs Throughout the Lifeline and
Link-Up programs, low-income households only has to
pay $10.65 for local service and $13.70 for
connec-tion charges per month, as compared to $24.74 for
local service and $42.71 for connection charges
paid by residential subscribers without subsidy,
respectively From these figures, we easily
recog-nize that the second condition is not easily
satis-fied In other words, low income households are
staying with the current provider due to facing
with high switching cost Another technical issue
of using VOIP telephone is the fact that it requires
users to have available high speed internet access
Most of low-income households are not able to
meet this requirement due to their budget
con-straint
Proposition 2: VOIP could be a substitute
for long-distance calls but complementary to
local services.
Feijo et al (2007) analyzed the possibility of
VOIP as a substitute for traditional telephony on
both sides including demand and supply However,
in this paper we emphasize the greater ability of
VOIP as a substitute for long-distance phones but
as complementary to local calls First, VOIP
con-sidered as a substitute for long-distance phones
including domestic and international phones is a
recent trend Again, the rapid increase in
inter-national call market of Skype is a very good
ex-ample Second, VOIP could be a complementary
to local services Obviously, an increase in volume
of VOIP services leads to a rise in the volume of
local telephone calls because almost of VOIP toll
telephone traffic must use the local telephone
net-works
Now, we consider more details of demand
func-tions and cost structures in the model This model
is expanded in the telecoms market with one
ILEC (Firm 1) and another VOIP provider (Firm
2)
The model with two providers has been
devel-oped by Bijl and Peitz (2004) They analyze a
sit-uation of one-way access Their model considers
the providers who are mainly different in terms
of the property of local-loop Firm 1 owns the local
loop network and the other does not Meanwhile,
in our model, Firm 2 may own a local loop net-work
The profit functions of Firm 1 and 2, are re-spectively as follows:
p1= l1Q1( p1, p2, q2( p2) ) * ( p1- A V C1) + ( 1
-l1)Q21(p1,p2,q2(p2)) * (r1-AVCr1)-FC1 (1) and
p2= l2Q1 2( p1, p2, q2( p2) ) * ( r1- A V Cr 2) + ( 1
-l2)Q2(p1,p2,q2(p2)) * (p2-AVC2)-FC2 (2) Where Q1(p1,p2,q2(p2)): quantity demand for Firm 1’s services
Q2(p1,p2,q2(p2)) : quantity demand for Firm 2’s services
Q21(p1,p2,q2(p2)) : quantity demand for Firm 2’s services accessing to Firm 1’s local loop networks
Q12(p1,p2,q2(p2)) : quantity demand for Firm 1’s services accessing to Firm 2’s networks
p1,p2: prices of Firm 1 and Firm 2, respectively
q2(p2): quality of services of Firm 2
l1: A proportion of consumers that are a “high type” using Firm 1’s services
1- l1: A proportion of consumers that are a “low type” using Firm 1’s services
l2: A proportion of consumers that are a “high type” using Firm 2’s services
1- l2:A proportion of consumers that are a “low type” using Firm 2’s services
AVC1, AVC2: average variable costs of Firm 1 and Firm 2, respectively
FC1, FC2: Fixed cost of Firm 1 and Firm 2, re-spectively
r1 and r2 : line rental charge of Firm 1’s net-works and Firm 2’s netnet-works, respectively
Intuitively, the demand for Firm 1’s services comprises two types of consumers The high-type consumers are willing to pay for high quality
Meanwhile, the low-type consumers are not will-ing to do so Without threat of entry, the incum-bent (Firm 1) will set a higher price to sell only
to high income consumers After appearance of the rival (Firm 2), the incumbent has to lower its prices to sell to both types The new entrant is playing an important role in changing the demand structure of Firm 1 Proportion l1decreases as the demand for Firm 2’s services increases At the cur-rent time, the incumbent dominates the local net-works, so it can set the line rental fee (also called interconnection charges; access charges or
Trang 5termi-nation charges) for its rival But it does not need
to purchase access charges from its rival This
case can be called the one-way access framework
However, in the future the incumbent might have
to pay the access charges for using its rival
net-work Moreover, each provider needs to invest in
the infrastructure system to provide its own
sub-scribers with better services As a result, each
provider has to interconnect with other providers
to ensure its consumers be able to call all users
This case is called the two-way access framework
The concept one-way and two-way access
frame-works are studied by many authors Armstrong
(2002) explains carefully those concepts
We now consider the high-type consumers with
the quantity of demand function (Q1(p1,p2,q2(p2))
In many other studies, the high-type consumers
can be explained in many different meanings such
as high demand, high value quality and so on In
this paper, we consider the high-type consumers
as the high-income users The demand of
high-in-come users for voice services depends not only on
the prices but also on the quality of services
Al-though, Firm 2 offers a very comparative price,
the number of users of Firm 1 seems not to be
fluc-tuated significantly since the quality services of
Firm 2 are unstable or insecure In other words,
the subscription demand for Firm 1 is not elastic
However, the quantity of demand for Firm 1 can
be affected by the new entrant, because many
users from Firm 1 can use service provided by
Firm 2 as a complementary service Therefore, the
prices and quality of services can affect on Q1 as
Intuitively, if quality of services (q2) increases,
the more Firm 1’s users will switch to Firm 2’s
services The demand of low-income consumers for
using Firm 1’s services has a positive relationship
with p2 and q2, but has a negative relationship
profit functions, the market shares of Firm 1 is
accounting for a higher proportion than Firm 2
since Firm 1 dominates the market for a long
time The market demand for using voice services
between two providers will depend on their prices
However, the FC1is higher than FC2, meanwhile
AVC1 is smaller than AVC2 Because Firm 2 is
willing to enter the voice market, it has to spend huge money on advertisement as well as the rental line For instance, Verizon is considered as Firm 1 (Vietnam Post and Telecommunication – VNPT for the case of Vietnam) and Vonage is Firm 2 (FPT Telecom for the case of Vietnam) Vonage’s strategy is obtaining the market share instead of the profitable objective at the move-ment For each new line, Vonage spent around
$239 on marketing cost (Wall Street Journal, Au-gust 02, 2006), while customers only have to pay
$24.99 per month plus 1 month free
Intuitively, the quality services of Firm 1 are more reliable and better than Firm 2, therefore p1 is more expensive than p2 for identical serv-ices Given p1, Firm 2 offers p2that is cheaper and more available services than Firm 1, such as un-limited calls in US, Canada and other European countries The rental fee or leased price ri(i=1,2)
is regulated by law, in current time, therefore r1
is more concerned than r2since Firm 1 wants to charge the rental fee Unfortunately, the rental price is often set by regulation, hence Firm 1 can-not raise network accessing fee as well as network interconnection charges to bar Firm 2 from enter-ing the market In future, when Firm 2’s networks broadly develop, it will require a line rental for using its networks
Both providers want to maximize their profit given a counterpart price By taking First Order Condition from Equation (1) & (2), we could find a unique pair (p1*,p2*) Hence, at a certain time, there exist two equilibrium prices in the voice market
Consumers are trading off between quality of services and prices For consumers with high in-come it is not necessarily best to switch from Firm
1 to Firm 2, but consumers who have a limited budget or emphasize cost reduction may consider switching to a new provider To sum up, the above model could be used to explain that VOIP service
is a substitute for long-distance phones but com-plementary to local services
4 Empirical study The data on telecommunication services rev-enue over the period 1995-2006 are collected from the Federal Communications Commission (FCC) The number of subscribers of fixed phones, cell phones and their corresponding prices are
Trang 6col-lected from FCC and CTIA-The Wireless
Associa-tion The price variables used in the analysis are
the average local monthly bill of cell phones and
average monthly bill of wire-line providers The
income per capita and population of each state are
collected from the US Bureau of Economic
Analy-sis (BEA) The data present greater aggregation
than would be desirable: they include all
sub-scribers and sales volumes but do not clarify the
more detailed sales of the telecommunications
services as well as pricing tariff for each state
In this study, we examine annual demand for
bundling telecommunication services across states
of the US over the period 1995-2006 To simplify
the model of telecommunication services demand
across states as well as to compromise with the
available data, we consider wire-line services and
wireless services as two major sources
contribut-ing to the demand for telecommunications
serv-ices Therefore, the empirical model can be
written as follows:
ln(Bst)= b0 + b1 ln(w_lessst)+ b2 ln (w_linest) +
b3ln(popst) + b4ln(incst) + b5INT + b6VOIP (1)
ln ( )= m0+ m1ln( ) + m2 ln( ) +
m3ln( ) + m4ln( ) + m5VOIP (2)
ln ( )= s0+ s1ln( ) + s2ln( ) +
s3ln( ) + s4ln( )+ s5VOIP (3)
where the variables in above equations are:
Bst: telecommunications revenues of state s in
year t
: telecommunications revenues of state s in
year t for consumer’s income per capita less than
$26,000
: telecommunications revenues of state s in
year t for consumer’s income per capita of at least
$26,000 w_linest : total expenditure on wire-line serv-ices of state s in year t
w_lessst: total expenditure on wireless services
of state s in year t incst: Consumer’s income per capita of state s
in year t popst : Population of state s in year t l: low personal income per capita less than
$26,000 h: high personal income per capita at least
$26,000 INT: refers to the dummy variable of internet access It equals 1 for internet access available after 1996 and 0 otherwise
VOIP refers to the dummy variable of Voice over internet protocol It equals 1 for VOIP service available after 2001 and 0 otherwise
b, a, m and s: coefficients of regression models, respectively
In Model (1), we consider the effect of wireless and wire-line services on the total telecommuni-cations sectors We use Model (1) to test our hy-pothesis that VOIP services have a negative effect
on the total telecommunications revenues
We use Model (2) and Model (3) to test our hy-pothesis that a low-income person has less oppor-tunities for using VOIP services than a high one
Here, we consider a low-income household who earns less than $26,000 per year The low-income figure is considered as equal to 75% of US
per-Bst i w less_ st i w line_ st i
pop st i inc st i
Bst n w less_ st n w line_ st n
pop st n inc st n
Bst i
Bst n
Constant -9.2722 (0.5046) - 9.0327 (0.7724) -9.3429 (0.7003)
ln(w_less) 0.1591 (0.0177) *** 0.1857 (0.0166) *** 0.1743 (0.0229) ***
ln(w_line) 0.1173 (0.0114) *** 0.0479 (0.0177) *** 0.1371 (0.0142) ***
ln(pop) 0.6969 (0.0224) *** 0.7439 (0.0234) *** 0.6627 (0.0287) ***
ln(inc) 0.4907 (0.0352) *** 0.4320 (0.0660) *** 0.5285 (0.0522) ***
INT 0.0365 (0.0158) ***
VOIP - 0.1478 (0.0161) *** -0.0816 (0.0236) *** -0.1720 (0.0204) ***
Table 1: Bundling telecommunications services regression (dependent variable: a translog bundling revenues)
Standard errors in parentheses next to coefficients are robust to heteroskedasticity * Significant at 10% ** Significant
at 5% *** Significant at 1%.
Trang 7sonal income in 2005.
Estimated results
Table 1 presents the estimates for our
empiri-cal models Model (1) includes two dummy
vari-ables, those are, INT and VOIP, reflecting the
effects of new technological adaptation on
devel-opment of telecommunication industry
As shown in Table 1, many parameter
esti-mates are consistent with a priority theoretical
expectation In order words, the wire-line and
wireless services have positive effect on the total
revenues The more income, the more demand of
new value added services that consumers are more
likely to reach The larger the population, the
higher the demand for telecommunication
serv-ices The internet access has played a significant
role in developing telecommunication sector
Par-ticularly, it created high demand for broadband
services and many value added services
through-out IP Finally, VOIP coefficients are negative
ones and those are also consistent with our first
hypothesis
Now, we examine the second hypothesis
Coef-ficient of VOIP in Model (2) is – 0.0816 meanwhile
that of Model (3) is – 0.1719 It means that a
high-income person has used VOIP much more than
that of a low one leading to a stronger negative
impact on the total telecommunication revenue
Notably, we also see that the coefficient of VOIP
in Model (1) is - 0.1478 ranging between the above
figures as a result of pooling all cross sectional
time-series data for all states and in period
1995-2006 Therefore, this has consolidated our
Propo-sition 1
The other significant result of empirical
analy-sis is that evolution of technological innovation
could lead to positive and negative effects on both
demand and supply side As shown in Table 2,
co-efficient of internet is positive, whereas that of
VOIP is negative The results show that VOIP
services have not been available for all consumers
VOIP service is rapidly emerging in the global
economy and it is changing many traditional
be-haviors of customers as well as social welfare For
instance, 10 years ago, almost consumers in
de-veloping countries were unable to make an
inter-national call to their relatives or friends in devel-oped countries due to high charges and that was why the call back or collect call services used to
be popular at that time However, with VOIP serv-ices, now many customers in developing countries can make an international call at a cheaper price than that of their counterparts in developed coun-tries VOIP services in particular and value added services based on IP will rapidly take off as new wireless networks such as Wi-Max
The new convergent trend of telecommunica-tion and digital television industries may lead to the new process of acquisition and merger be-tween both sectors Meanwhile, VOIP services seem to be diversified in both software and hard-ware terms so as to improve services and create more demand For instance, Google Voice becomes
a very strong VOIP provider competing with sim-ilar services such as Skype When it introduces a free service that gives callers a new phone number that can access to different normal phones they own and send them an email transcript of voice-mails they received (Wall Street Journal, Aug 28, 2010)
5 VOIP services in Vietnam
By law whenever customers in Vietnam would like to use cheaper long-distance calls via VOIP, they have to access local providers by using fixed line phones or mobile phones The main providers include VNPT (dial 171), Saigon Postel (dial 177), Viettel (dial 178), EVN Telecom (dial 179) and FPT Telecom (dial 176) Recently, the market shares of VOIP services account approximately for 60% of total long-distance calls in Vietnam (Vn-Media, 10 April, 2008) However, the internet users have not been accessed to all VOIP services offered by internet providers such as Google or Yahoo voice in Vietnam like in the US because of current regulations Obviously, VOIP in Vietnam
is really a substitute for long-distance calls but complementary to local services This is the same situation as we find in the US
6 Conclusion The VOIP services have truly created the new game in the telecom market VOIP services are rapidly emerging as a result of successful
Trang 8implica-tion of innovative technology and software as well
as business strategies This paper introduces the
dynamic internet telephone market and its
im-pacts on the voice market It provides some
evi-dences (1) a low income person has less
opportunities for using VOIP services than a high
one; (2) VOIP services could be the greater
possi-bility of VOIP as a substitute for long-distance
calls but as a complementary to local services
Specifically, the results of empirical analysis also
consolidate the hypotheses and drive some other
significant results such as evolution of
technolo-gies could have both positive and negative effects
on development of telecommunications industry
and market segments
There are some limitations that need further
investigating First of all, we have not accessed
the current data of all states of the US Secondly,
our model is able to explain the current situation
of VOIP providers in Vietnam, but we have not
found the data to test the hypotheses These
is-sues are challenges for future researchn
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