electronista reported the following great news:
Google Voice users learned late Monday that the service now has a way of making purely Internet-based phone calls. Making a SIP call with a “sip:” prefix, the Google Voice phone number and @sip.voice.google.com skips the conventional phone network entirely, saving users cellphone minutes.
Disruptive Telephony tested it and found that a call worked “great.”
The development is relatively new and, according to Google’s partner YATE (Yet Another Telephony Engine), was added on January 31 without any public mention.