I use a sip account thru pbxes.org linked to a us number thru ipkall.com along with sipdroid, voice+ and google voice. Totaly free incomimg and outgoing us calls thru your google voice number. I know it sounds complicated, and kind of is but its worth it in the end.
Once setup when you dial a number voice plus catches the call and asks whether to use regular phone or a "callback" from google voice. So you select callback which sends the info thru the net to google voice who then calls your free sip number. You answer and are automaticly connected to the number you dialed for free. Then on the google voice settings page you have your gv number forwarded to your free sip number and all incoming calls are free.
On a side note stock roms and probably most custom roms do not allow sip over 3g by default. You must edit the framework-res.apk file to allow it.