Google Voice Now Offers SIP Addresses For Calling Directly Over IP 15 years ago
We found today this is no longer the case --> http://www.onsip.com/blog/rob/2011/03/08/google-voice-sip-ad...
As for Yate, we believe it has the same issue we found with native Android SIP support: It doesn't distinguish between auth username and username, which creates limitations. (SIP Authentication username not configurable - RFC 3261 Section 22.1)