The problem is not with unlocking the device which is possible. After that he device can be used with other VoIP providers that use SIP.
Google Voice however uses a different signalling protocol (XMPP), it's not SIP like the majority of VoIP providers.--
Providers (through asterisk): voip.ms, freephoneline, smartcall.ro. Hardware: Vonage VDV21, Moto VT2x42, Linksys SPA series, Grandstream HT series, Panasonic KX-TGP5x0