Peon, did you ever get to the bottom of this?
I'm also using 3G with an IP phone, and have discovered that incoming call issues can be related to what mode the 3G router is in. In idle mode (WCDMA) if the signal is poor, the invite packets aren't responded to quick enough and the call goes to voicemail, even thought the phone sometimes rings. When the router switches to HSPA, the latency is good and all works as it should.
Really, the invites should have a longer timeout, I understand up to 32 seconds, but that's not what I'm experiencing.