Good information from HansCT, but I will chime in and suggest you give Skype a try. I believe they have vastly improved the quality of the service recently. I have not used their service that...
So bottom line if call quality is your hot button: use AT&T.
If in your neck of the woods VZW is the way to go, investigate bypassing their voice services and go with VoIP?
If you want good, forget free.
First figure out how to do VoIP right - it's a bit of a research project. Browsing the DSL Reports archives is a great help.
Along the way you'll realize when...