UK vendor Truphone has demonstrated software that lets Apple iPhone users to make low-cost phone calls via the Internet.

During the demonstration at the Demofall show in San Diego mid-week, the company showed how its software could be used to make a call from an iPhone, routed wirelessly to its Internet servers, and then back again through the same access point to either a second iPhone or a mobile handset. The technology does this without interfering with the SIM or phone subscription in any way, the company said.

The software uses a combination of Voice over IP (VoIP) and the Apple device’s built-in Wi-Fi capability to route calls via the Internet whenever users come within range of a wireless access point, allowing them to bypass the expensive mobile tariffs that are being sold with the iPhone in favour of VoIP call rates.

Altogether, it can manage session initiation protocol (SIP) to SIP, SIP to a phone, and phone to SIP.

The catches are that the software does not yet exist as a working product – it is said to be in development – and the call setup might take slightly longer than a conventional mobile call. Truphone’s client, launched in May 2006, already runs on Nokia’s E-series Symbian handsets.

"Truphone is truly the mobile operator for the Internet age and our aim is to see Truphone available on all cellular phones with the inherent functionality needed to make VoIP calls. The Apple iPhone is such a phone and we're pleased to be able to show it working for real," said Truphone CEO James Tagg in advance of the demo.

The company also showed its technology could be integrated with social networking website, Facebook, allowing users to share a Truphone ‘call me’ button with friends while keeping their actual number a secret. In the company’s view, adding communication possibilities to such sites represents the highly-sociable future of telephony. This application is also in development on an unknown timescale.

There are currently no official clients that will allow VoIP on the iPhone, though workarounds for Skype have been in circulation. Truphone would make a name for itself if it manages to reach this particular landmark with a shipping product.

A YouTube video of the product demonstration can be seen on the company’s blog.