Google Voice Discovered Allowing Pure VoIP Calls
From the article "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.'"
If you take an android device with google voice installed. Tell it use google voice for all calls. The get an Xlink device (http://www.myxlink.com/index.aspx). Peer the XLink to your android device via bluetooth. Now you have analog dial tone coming out of the XLink and you can put it into a PBX or regular analog phones.