You may take a look at siphon (http://code.google.com/p/siphon/).
From their homepage:
Home of the World’s first free
SIP/VoIP application for iPhone and
iPod Touch 1 and 2.Siphon SIP/VoIP project is the first
in his category that works on iPhone
and iPod Touch 2 with headset for all
SIP providers. It is a native
application approved running on 2.X
using internal micro/speaker and
headset.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 applications.