Sipgate is a free download that bring VoIP to the Apple iPhone whereby you can use sipgate to place and receive calls over WiFI using any SIP based VoIP service.

With the sipgate application on your Apple iPhone, iPhoners can bypass roaming charges, save mobile minutes whenever WiFi is available, and even place calls at virtually no cost while travelling aboard.

However the iPhoner will need to purchase a Wi-Fi connection from third parties

At the present time sipgate is only available for firmware version 1.1.4, so at the moment will only work on the first generation Apple iPhone.

Source — sipgate