According to Truphone, they have found a way to unlock IP telephony on the iPhone, and demonstrated it today at DEMOfall 07 in San Diego. James Tagg (CEO Truphone) was able to call another mobile phone using only an IP connection w/o breaking the SIM lock. The used software is still in beta version according to Truphone, but there are plans to have a version with simple activation soon, and allow seamless switching between SIM and WLAN use. Definitely, a great development for iPhone users, but a complicated situation for AT&T. Check the video after the break to see it all in action.

(more details: www.prnewswire.com)

those of you prefering uncomplicated VoIP telephony check this:
Siemens Gigaset VoIP phones