Truphone is quietly testing early stage development software that will enable VoIP calls on the iPhone - and doesn't require Apple's device to be unlocked.

Use of the software will enable iPhone users to make calls at significantly lower rates than those charged by the mobile networks - even when making international calls, or when in countries other than their home state.

TruPhone hasn't taken its software to final release candidate status as yet - it expects to issue a strictly limited beta version of the software in about four weeks.

At present the solution requires users to access the Terminal application hidden somewhere deep in the iPhone in order to tell the device to send a call over WiFi, rather than through the built-in SIM card.

Truphone promises the final release of its solution will be consumer-friendly, according to a report on BlogNation USA. The solution will enable the Apple device to switch seamlessly between VoIP over WiFi and the SIM.