I have two GV numbers.

One points to my sipgate account, google talk, cell phone, work phone. sipsorcery used to call out.
The other just uses google talk. A friend has Asterisk running, which logs in to google talk for me.

I have a Linksys PAP2 device that logs into both accounts for me, and i can make/receive free phone calls.

FWIW, and YMMV, the second option is more reliable, but means giving up google chat from that account (inconvenient to use another account, as voice.google.com wouldn't be as easy to get to).
The first option relies on sipgate stability, which isn't always perfect. Lately, i've been having issues (i can't hear them, phone doesn't pick up, etc.), and i have to reboot the PAP2 or whatever to make it work.