i'm currently working on the same project. I'm trying to realize all this using java witch posses some huge problems as it seems that RTP is not supported by Java. Also getting SIP to work in the Sun WTK seems quite tricky..
please keep me (the forum) updated with your findings.
ok, i made some progress. Getting SIP to work in the WTK means you have to recode the Nokia SIP package. (is not easy, and i suppose nokia wouldn't realy like it)
RTP isn't supported by any phone (maybe the new nokia communicator).
AMR works just fine, but not in the emulator, so you can only test this on a actual device..