C turned me on to Blink, a very high quality SIP communications package, which just happens to be GNU General Public Licensed, v3. With Android 2.2 and Google Voice I’ve been forced to use a mix-mash of modes to make cSipSimple work reliably. Part of the issue is forwarding phones. With Google Voice I am now forwarding to Gmail, Virgin Mobile, and now SIPgate One. When I turn off all cellular communcations in theory cSipSimple should be the only phone to ring, and in theory it shouldn’t use mobile anytime minutes.
I managed to get this to work on the PEAP encrypted 802.1x Wifi on campus, so in theory at home it should work fine. ICE/STUN with NAT seems to help call quality. The SIPGate iOS application also seems to work fine, though sometimes there is a bit of crackling not present on SIP>SIP calls using other clients.
A third option, which may or may not be in the works at the Googleplex [sic] is the integration of SIP directly to Google Voice. Google was likely testing this functionality as Dan York wrote in his blog, however, it appears Google pulled the plug on it publicly. It would be incredibly convenient to not have to route via the PSTN for Google Voice because as Dan mentions, the calls are already IP based, if only they could stay that way Google Voice users would have higher quality calls and an easier time of it. To test if you have any SIP functionality, plug in +email@example.com, where that number is your GV number.