While T-mobile’s SIM in the USA is a good deal especially if you plan to re-use it, nobody offers good overseas calling from most cell plans.

Those who do a lot of overseas calling look for a dial around, like rebtel or talkplus or one of its ilk. You dial a local number (just your ordinary airtime rate of 10 to 20 cents) and they forward overseas for you using voip for a couple of cents more.