TMobile is a bit harder to navigate - but with my limited Czech I think their rates are significantly higher.

Vodaphone's site is probably the easiest to navigate, but their rates are about 2 bucks a minute if you call the US from abroad.

Another option to all of this is to get a PDA with a WiFi card and call the US using Skype, which is free 'till the end of the year and only 2 cents after that.

Of course wifi's not everywhere...