I've found the U.S. carriers plans to be prohibitively expensive for that. Outside of wifi coverage, I sometimes use my unlocked phone with a sim card, for local calls in the countries I most frequent.
Probably not what you are looking for, but you might check into skype "wifi", similar to skype, but allows you to connect to local carriers (Singtel, China Tellcom, HK Telcom, etc..) wifi signals for a small fee, 4 to 7 cents a minute or so, and the usage is deducted from your regular skype account. Works pretty slick.