Let me explain what I do...
The woman and I both have AT&T Prepaid. GoPhones.
We buy one $100 card a year from CallingMart for $90. So the 10c a minute plan comes out to 9c a minute effectively.
We install Google Voice and GrooveIP on our smartphones to make VoIP calls.
Our smartphones are Samsung Exhibit II which I paid full price for, they're about $150-180. Fast enough for just about anything except the most graphical games. Since I paid for them, I was able to get T-Mobile to unlock them without a problem. I use them on AT&T's network now.
Let me know if you have any questions about our setup. Using a GVoice number, we effectively have free talk time anywhere we have a wifi signal, including at home. There are some technical details of course.