Mobile VoIP player Truphone has opened its system to free calls to Google Talk, linking instant messaging to VoIP.

Truphone, launched last year, is free software that turns Wi-Fi-equipped Nokia mobile phones such as the E61, into dual-mode phones that can make inexpensive calls over the Internet using Wi-Fi.

When Truphone users get within range of a Wi-Fi service, their phone switches over VoIP. They also pop up as "online" to their friends on Google Talk, so it is clear to other people when they can make a free call to them. Calls between the two services use the standard XMPP protocol and are free. Truphone has put a video on YouTube of the service in action

The service will appeal to individual users, to those wanting to test VoIP and IM convergence out, and to those not under a tight rein from their IT department. It may set some alarm bells ringing for others, if any large scale use is considered. Unplanned VoIP over the office wireless LAN may cause bandwidth problems, and corporations are already starting to worry about the blossoming of IM, which can operate outside corporate governance

Truphone is free, as are calls to Truphone and Google Talk. Calls to other numbers are at a discount over regular phone rates -- currently with an offer of free calls to 40 countries from the United Kingdom, and free calls within the United States and Canada for all U.S. customers. Customers get a Truphone number for inbound calls, and can forward all these calls to the GSM number on their phone.

While currently only available on Nokia E-series phones, Truphone has promised software for other handsets including Windows Mobile devices soon.