1) depends on the bands your phone covers. Our GSM here is mostly 850 or 1900. The rest of the world is of course a bit different.

2) Not sure about prepaid Sims

3) State to state and even state overseas, I use onesuite for all my long distance calls. As low as 2.5 cents per minute, with $10 minimum purchase. Full details at www.onesuite.com. I've been using them for years.