I'm not sure on PR1.3, but I was doing this sucessfully prior to 1.3. GSM first then SIP (Gizmo5), answering would answer the SIP call. Worked on both wifi and 3G data (on TMo US).

I assume SIP works when only that number is dialed?

I asked about before the N900 came out. Everyone in the know said don't worry, won't be a problem. I really wish you could pick a call to answer.