Depending on what country you are visiting, I recommend buying a $20 phone over there. I pay 10 pence per minute for calls back to the USA using "pay as you go" service from Tesco, which is like WalMart over there. That's a pretty good rate. You "top off" your account in increments like 5 or 10 pounds (or 5 or 10 euros) at thousands of stores (not just Tesco stores - it can include neighborhood markets). I bought my phone in England and have used it in Germany and Hungary as well.

The only downside: I could not top off from the USA because their on-line service did not accept American credit cards as of a couple years ago.