I'm using
Voipo. I prepaid for two years for $149, installed it myelf (pretty easy, just plug it into your router), so I saved about $50 month (I have no monthly landline bill at all now). But VOIP, or Voice Over Internet Protocol, can be sketchy sometimes. I get people who end up going straight to voicemail (on their server) when the cable internet lines are crowded.
I'm not sure if Vonage would have the same problem or not, though.
Here's a review of VOIP services