Yes it's true; a company called Truphone was able to make a call from a ?locked iPhone? using an IP connection established through a Wifi connection. The fact that iPhone users will now be able to make VoIP calls whenever they are within a Hot Spot is incredible news!
According to CEO James Tagg when the final version of the software is released users will be able to switch between Wifi and SIM calls without interruption.
You can see a demonstration of the VoIP call here