A new crop of start-ups offers bypass calling using unlimited data plans. If the past is prelude, they could be the next Skype or Vonage.

It was only a matter of time before calling services à la Skype and Vonage came to the wireless world. You wouldn't know it judging by mainstream media and marketing, but a slew of start-up companies have launched wireless voice-over-IP services and are quietly gaining a following. Like their predecessors in the wireline world, these new companies help customers avoid or minimize traditional voice fees — typically by using Wi-Fi or cellular data service to carry calls in IP format.

Fring, iSkoot, Mobivox and Truphone are just a few of the companies that have entered this market — and although they aren't exactly household names today, we're sure to hear more about them moving forward.

TRUPHONE'S APPROACH is most similar to that of Vonage in the wireline world. Like Vonage, Truphone uses a digital connection to send calls over an IP network as an alternative to making a traditional circuit-switched call. To use the Truphone service, customers download a software application onto their dual-mode Wi-Fi/cellular handset. The software enables customers to place domestic and international calls to mobile or landline phones at reduced rates.

“It's a fully integrated experience,” said James Tagg, CEO of Truphone. “People can use their contact list or call log to call someone back.”

Calls are carried in IP format to a gateway operated by Truphone, where they are routed to the call recipient, using circuit-switched infrastructure at the terminating end if necessary. Companies like Vonage paved the way for companies like Truphone by negotiating deals with competitive carriers around the globe to create a wholesale VoIP termination market at rates that are considerably lower than those offered by traditional carriers.

About 90% of Truphone customers use Wi-Fi to make their calls, thereby eliminating cellular charges, Tagg said. When Wi-Fi is not available, the same service can work over 3G data service. In the U.S., most carriers offer unlimited data service for a flat fee — and in many other industrialized nations, at least one carrier offers such a plan.

Another similarity between Vonage and Truphone is that Truphone users in North America and the U.K. get a separate phone number that works independently from their traditional phone service so people can place calls to them over the Truphone infrastructure. In the other 40 countries where Truphone offers service, the Truphone software presents the traditional cell phone number to call recipients who may want to do one-button callbacks.

About 30% of Truphone users are in the U.K., but the company's next largest market is the U.S., which currently contributes 20% of Truphone's business. Although only a handful of U.S. operators support dual-mode devices, Tagg noted that about three-quarters of Truphone customers buy their handsets from someone other than their traditional operator, moving a SIM card from a low-cost, carrier-supplied handset if necessary.

“The next step in our business model is to become a full operator in the U.S., and people will come to us for the handset,” Tagg said. Currently the company can do cellular/Wi-Fi handoffs outside of the call.

Tagg claimed 25,000 active Truphone users. “That's not a download number,” he said. “These are people who are actively using the service to make paying transactions.”

Customers pre-pay with a credit card, then draw from their account on a pay-as-you-go basis. At press time, Truphone was poised to launch a monthly post-pay plan as well.