Besides free call, you can also text for free (using Talkatone or the Google Voice app) and not pay the ATT $20 texting fee. Choosing a very easy phone number that everyone can remember is a good idea and not having to give out your real one. This was what I did for two of my friends:
-ATT 3G + WiFi, iPhone, Talkatone:
I haven't had any complains with this setup, probably since the person always get good signal and stay in one place with WiFi signal. You have to dial within the Talkatone app, but it integrate really well with the iPhone, so you don't miss incoming calls. It has its own call logs and texting window. I much prefer it over GrooVe IP and it seems to work good on iPad too. I have not used the Android version of Talkatone though.
-T-Mobile 3G, Android, GrooVe IP:
This one was a fail. Since T-Mobile probably doesn't have best 3G signal penetration so the person keeps losing signal. Also there can be issues with echo, I don't know how bad this issue compared with Talkatone. Also you have to remember to set WiFi to never turn off or in GrooVe IP option set it to keep screen on during calls. I don't entirely blame the app, but I guess its more to do with getting good data signals.
I wouldn't recommending choosing this option primarily, I would rather refer to the
prepaid thread and check out Simple Mobile or Straight Talk first. Overall, I think if you have strong signal or remain stationary in a WiFi area then its good.