Siphon SIP -VoIP for iPhone and iPod Touch.

Siphon SIP/VoIP project is the first in his category that works both on iPhone and iPod Touch. It is a native application approved running on 1.1.X using the Microphone built-in headset, and an external Microphone (for example, designed by the Touchmods team) for the iPod Touch.

The Application supports the SIP standard, preserving compatibility with hundreds of SIP providers and offers a GUI which preserves the apple design of native iPhone and iPod Touch applications.

http://code.google.com/p/siphon/