Mobile VoIP is set to grow, but it will run over the 3G data provided by cellular handsets, rather than over Wi-Fi, according to a research report from Disruptive Analysis, which predicts 250 million users of 3G VoIP by 2012, compared with less than 100 million for voice on Wi-Fi.

"Yes, 250 million is a surprisingly big number," said Dean Bubley of Disruptive Analysis. "I did the sums and then had to triple-check my own model. The fundamental truth, he says, is that mobile networks are moving towards LTE (or possibly UMB) and that will be all IP, so for operators "VoIP is mandatory."

VoIP will allow carriers to handle more calls on their scarce spectrum and backhaul, and reduce expenses by handling all traffic as data. It will also let them offer new services such as push-to-talk and voice-integrated "mashups", says Bubley, in his report, VoIPo3G Business Models.

VoIP on 3G also fits the move to femtocells, which use the subscribers' broadband service to increase coverage in the home, since the digitized voice is ready to be handled by the femto's broadband backhaul.

However, because new radio technologies are not yet widespread, we can look forward to a few more years of the current confusion while third party providers such as Truphone and EQO (and many others) offer mobile VoIP using Wi-Fi services or the user's data plan. "Some independent VoIP players are already exploiting the fact that today's 3G networks can already support VoIP, putting dedicated software on smartphones, exploiting open operating systems, flat-rate data plans and features like 'naked SIP' and built-in VoIP capability," said Bubley.

The rise of mobile data in laptops, either as built in modems or data cards will also contribute to the growth of VoIP on 3G, as this service often competes with home broadband, and users expect to be able to use familiar applications such as Skype or other telephony software. "Some operators are even offering their own VoIP software for PCs with wireless broadband," said Bubley.

"By 2012, most VoIPo3G users will be using mobile carriers' own standards-based VoIP capabilities, over the new, advanced 3G+ networks," said Bubley. Only 60 million of them will still be on the independent operators' offerings. "The key thing to think about is that for many users, VoIP will be invisible."

And there will still be more to go, because 250 million will only be about ten percent of mobile phone users, and 20 percent of 3G+ users.