I have been wrestling with this before.
I have a Nokia E60 and that works great as SIP client.
However i have created an extension with the GSM number of the same device.
In the ring group i have routed calls to my GSM extension when the SIP clients are not reacheable. This works, the phone receives the incoming call over GSM but I don't have any audio at all. I have tried several options like Audio Bypass, but nothing works...