Billing itself as a "4G" service, new operator Truphone has announced a Wi-Fi based telephone service that uses emerging Wi-Fi enabled handsets such as Nokia's E-series to offer cheap calls from public hotspots.

The service, to be launched in beta at next week's VON show in Sweden, handles voice over the phone's SIP stack, connecting over public hotspots, office Wi-Fi and home networks. Commercial launch is due some time later this year.

“Wi-Fi telephony is going to change users’ mobile phone experiences dramatically," said Truphone chief executive James Tagg. Truphone is considering MVNO deals which would allow it to offer the mobile part of the service as well.

Truphone has written its own client application, which varies according to the level of provision on the handset. "On some phones, there's nothing - no SIP, no network," says Tagg, "in which case we build the whole stack." Users will download the client software for their particular device form the Truphone ( site, which will go live next week.

Calls out to the conventional phone network will be managed like SkypeOut or those from other VOIP operators, with beta testers all given a £5-credit to get them started. When users move away from Wi-Fi, calls needed will be handed over to the mobile network seamlessly, Tagg promised.

Logging into public hotspots for voice on Wi-Fi has been tricky in the past, since the phone has to manage an authentication process designed for a laptop with a full screen, mouse and keyboard. Truphone will set up reseller deals with "major hotspot providers", to be announced at VON. The user will pay Truphone to get VOIP access to public hotspot networks, where the log-in will be automated, and Internet dial-tone should be pretty much instantaneous.

So far, the service is only tested on Nokia’s E-series handsets, but Tagg says others are coming soon and "Wi-Fi functionality on mobile phones will rapidly become commonplace." Truphone promises to have VoIP applications for other smartphones, so there will be mass market availability of low-cost mobile phone calls by the end of 2006.

As well as offices and public hotspots, Truphone wants to use the subscribers' home Wi-Fi networks: "Each new person that subscribes to the Truphone service becomes a mini-cell in a new, collaborative network that they share with their friends," says the release, "spontaneously building the world’s first 4G network." In practice, this will mean allowing friends who also have Truphone to use their home network to make calls. "It's the equivalent of walking into an office and asking to use the phone," says Tagg.

Like Skype or other VoIP services, calls will be free to other Truphone users, and offers reduced costs if one leg of the call goes over traditional phone networks. The service will be available as a pre-pay service, or with a traditional bill.