In this 2009 Google forum, a Google Voice user asked, "Does my Google Voice number have a SIP number?" Many answers followed with, "no," and alternative solutions - mainly involving Gizmo5. Today, we were referred to a blog by Todd Vierling that indicate Google Voice numbers have a SIP URI.
"The good news is, you can now call Google Voice numbers directly from SIP using the pattern +1NPANXXXXXX@sip.voice.google.com. This is a major step forward," describes Rao in his blog, entitled So Google Voice: SIP is Actually Coming (...in some form).
Based on this, I tried dialing my Google Voice number SIP URI, and it worked. This is a great find, but let's back up a minute. Vierling mentions his discovery is just after the Google announcement that they are discontinuing Gizmo5 service, covered in this Engadget blog. For some users, this poses a problem, which is well-described by Alok Saboo,on TrueVoipBuzz.com:
"Up until recently, Google offered the ability to forward the Google Voice number to a Gizmo account, providing some way to extend the reach of GV. However, Google announced the termination of Gizmo5 service starting April 3, 2011. For many of us, using Gizmo5 to use Google Voice services on ATA or other soft phones, this is a huge blow," says Saboo.
So at this moment, those who used Gizmo5 to register SIP devices and take advantage of Google Voice services are in a predicament; they are losing their SIP addresses and cannot register phones. While it appears SIP is in the near future for Google Voice, indicated by their GV number SIP URI's, this isn't of great help because users can't register a SIP device with these GV SIP addresses.
Since this is our area of specialty, we're going to make a few recommendations. Stay tuned.