Try: www.voipbuster.com. If you have an unlocked ATA like the Sipura 2100 or something like that: 1 EU, one time, and you can call anywhere in USA and Canada for free. Only downside: no incoming calls.

You can test the connection for free but are limited to 2 minutes per call, but US$1.5 is not bad for an unlimited outgoing connection! I have it as my "emergency" line.

Dave