We may have just seen the reasoning behind Google's announcement last week that they were shutting down VoIP service Gizmo5. Apparently a SIP address has surfaced that allows those with an SIP-to-SIP account (such as sip2sip.info) to call Google Voice users for free. While this may not sound Earth-shattering at first, this is most likely an early sign that true Google Voice VoIP (that many users have been eagerly anticipating) is on its way.
Say you are wanting to contact your friend, whose Google Voice Number is (555)123-4567. You could now sign up for a free SIP to SIP service, register your account via an SIP app (such as csipsimple or sipdroid), and contact +email@example.com. Doing this would call your friend's Google Voice number at no cost. The sip.voice.google.com suffix was not previously known to work in any capacity.
Many find the carriers' system of charging for minutes + data + text messaging to be an outdated model that preys upon customers' leftover expectations from the early days of cell phones, which established voice minutes as the central figure. As data becomes, more and more, all anybody needs to use a smartphone, some see VoIP as the future of mobile voice calling, and the folks at Google seem to be among that crowd.
I, for one, welcome this.