SIPSorcery looks useful, based on what i've pieced together so far. The web site could use a little less (or more) emphasis on the Silverlight UI (depending on the vision for the project). edit: i need to read the sipsorcery docs. it's not clear to me how sipsorcery helps in placing the call from a wifi-only android device, for example.

I just played around with this last night, and it seems like SipSorcery, through the Ruby dial plans, have hacked up a way to make the sip call appear to come from your GV number.

It works for me (though I used Sipgate since you can't sign up for gizmo5 anymore). The only problem I had was that when people call my GV number, the Caller ID when sipdroid rings says that it's from my own Sipgate number, which the original number displayed in brackets. (And perhaps that when you have both a sim number and the sipdroid one, both the normal phone app and sipdroid rings at the same time, so you have to make sure you answer the right way...)