I'm surprised no one has mentioned Voipo. I paid $165 for 2 years (~$6.80/month) 3 months ago (includes taxes and fees), ported the number from AT&T POTS, and wish I had done it years ago. They sent me a Grandstream ATA which works fine behind my DIR-655 router. Their service also includes tons of features plus 60 mins of international calling a month free. Hell, I was paying AT&T $9.99/month for just caller ID with, (drum roll here), name display.
I now love to get calls from Telemarketers. I tell them that I want to contribute/purchase and to call me back in a few minutes because someone is at the door. Then I go to the Voipo internet site and call route their number to a busy signal.