I have tested the Communication between MS Portrait and Sip Communicator
but I see that the can't speak, probably Portrait (there isn't the
source code) use a different RFC.
After this I have found a Client SIP for PDA (this is my target), this
is SJ Phone product of the SJ Labs, is freeware but give some problem
with the RTP.
For some information about my work send me a e-mail.