I use Google Voice; that's the number I give out to everyone. When I am traveling abroad I just get a local SIM and stick it in my factory-unlocked iPhone.
For free outgoing calls back to the USA I use Google Hangouts, VoIP over the local carrier's cellular data or if available, a WiFi connection. For free incoming calls from the USA, I have Google Voice forward to my free Talkatone number. For free SMS, I just use the Google Voice app.
Another option is to have Google Voice forward to a Google Chat (a.k.a. Google Talk) account which will ring your Google Hangouts app. Since Google Voice's recent refresh, I don't know how to configure this in the new interface. However, if you go to the Legacy Settings in the web browser version of Google Voice, you can find the setting in the old interface.
Sure, I can use the phone number from the local carrier's SIM to place local calls (or text someone locally), but the main value of using a local carrier's SIM is cheap cellular data, not voice minutes/SMS on the local network.
Far cheaper than paying $10/day to AT&T.