iCall has a closed beta of their system for the iPhone in work.

iCall allows you to route your incoming cellular network calls to your iPhone via VoIP when you are connected to a WiFi hotspot. You can also send and receive VoIP calls over WiFi.

Watch the demonstration video for more information.

Robert Poe for VoIP-News has written a handy article on how to get started making calls using the JAJAH software and network on your mobile phone.

JAJAH’s mobile VoIP service offers a ringback-and-connect type service which is described in the article.