Truphone has opened a new front in its fractious battle to undercut mobile networks, announcing a beta version of its software that lets Blackberry users make VoIP phone calls.

The company has built its reputation on its free Nokia/Symbian client, but from this week Blackberry users will also be able to use Truphone Anywhere to take advantage of international call rates that undercut mainstream GSM networks.

These vary by destination, but typically start at around 3 pence (or 6 cents) per minute for calls to most European countries and the US, considerably below that charged by mobile networks.

As long as the application is running on the phone, call routing dials via Truphone automatically, using the GSM network only to reach the company's point of presence, beyond which it is sent over the Internet. This local call bumps up the call rate slightly. Users can also decide to dial a number by making a conventional ‘full cost' GSM call if desired.

In contrast to the established version for Nokia/Symbian, however, the Blackberry incarnation is VoIP only, and can't be used to make totally free calls by connecting to an available Wi-Fi access point.

This was one of the software's most intriguing features, and coupled to the VoIP capability, upset one network, T-Mobile, to such an extent that the latter was accused by Truphone of blocking its calls. Truphone was eventually granted a court injunction against T-Mobile in 2007, which forced the carrier to route its calls.

According to a Truphone statement, it hoped to provide Wi-Fi support in future but that would require more development time.

"Building a Truphone Wi-Fi VoIP product for BlackBerry is something we intend to do. However, building such a product for the BlackBerry platform does pose challenges some of which would need access to parts of the BlackBerry OS that we currently do not have access to. We're looking forward to working with BlackBerry on this in the future though."

For the time being, the company is appealing to users using cost arguments.

"There's no GSM business tariff that gets close to the prices we can offer BlackBerry users with Truphone for international calling," said Truphone CEO, Geraldine Wilson.

"And in these days of financial belt-tightening, businesses are looking at every means of cutting costs, which is an opportunity for us. Truphone is a genuine alternative carrier for international calls, with the potential to reduce annual mobile bills for the largest companies by many millions of dollars," she said.

The company also offers a free VoIP client for Apple iPhone users - which works using Wi-Fi-only - and has plans to expand support to new platforms in the future, including Samsung, LG, Motorola, Sony-Ericsson and HTC.