How to use Google Voice™ on your regular phone and make calls to the USA & Canada for FREE ?
The easiest way is to buy an OBi110 adaptor: http://www.obihai.com/googlevoice.html
And it could be FREE if you already have a PAP2 ATA.
Try Google Voice-SIP gateway solution provided by Bill Simon.
https://simonics.com/gvgw/
After filling out the form, following the prompts, GV Gateway will test your account and then present SIP authentication and registration information that you will use to set up a SIP trunk to the gateway.
Notes：
1. You need a Google Voice account (Get a Google Voice Free phone number, unlimited US and Canada calling)
2. You need to enter Google credentials during the registration. If you are not comfortable using your account on the gateway, you can set up dedicated Google accounts, not tied to your e-mail or other services, just for Google Voice.
3. If you have never made a call through Google Voice before, you must first log into Gmail chat with your account and initiate a call through the chat widget. Then log into Google Voice, go to your voice settings and select Google Chat as the destination for call forwarding.