The number climbs to just under 17.5 million users when 'soft-clients' like Skype and VoiceGlo are added in, according to John Bosnell, senior analyst at Point Topic.
"Japan is still the largest market by some way, [as} Yahoo Japan alone has more than 4.5 million VoIP subscribers," said Bosnell.
After Japan, the American cable sector is the most important, with around 2.1 million subscribers.
Bosnell says the major Canadian cable companies all have VoIP plans. Shaw launched in February 2005, with Rogers and Cogeco planning for the middle and end of the year respectively.
Point Topic says the figures based on its broadband market research on behalf of the DSL Forum, publicly available statistics and estimates where figures were not available suggest US telcos are beginning to enter the VoIP market, with Verizon's VoiceWing service up and running, SBC rolling out services, and Qwest planning a launch soon.
These developments are partly driven by new network build, such as SBC's investment in fibre. However, Bosnell suggests they are also a response to Vonage poaching voice customers from DSL subscribers.
Vonage is said to have grown its subscriber base by almost 38 percent in the first 3 months of 2005.
"France is the most significant market in Europe at the moment, with over 1.2 million subscribers to retail VoIP services by the end of Q1 2005," adds Bosnell.
The Point Topic research also looked at the phenomenon that is Skype, but concluded it is difficult to ascertain just how many people use the service.
The researchers suggest Skype had 100 million downloads by mid-April 2005. Of these, 35 million users had registered, and there were 1.2 million SkypeOut accounts.
Skype does not publish subscriber numbers, but it does provide a rolling ticker showing how many minutes it has served. By monitoring this, Point Topic gauged a weekly and daily average of minutes served.
"Dividing by a 'rule of thumb' number of say 7 minutes per day then gives a feel for Skype usage. Doing this for 2 weeks in June 2005 yields an average of 5.6 or 5.3 million 'user-equivalents' for the 2 weeks."
This suggests that many people download the client and do not register, and many people who register never or rarely use Skype, a VoIP soft-client version of the 80:20 rule perhaps.
"But combined with our number for VoiceGlo we estimate that soft client services came to around 5.9 million VoIP users in Q1 2005," said Bosnell.