Are you using the prepaid SIM with a device you purchased from at&t? If so, I would visit your local Corporate store for assistance so you can make certain everything is in working order before leaving the store.
If you are using the prepaid SIM with a device that you provided - is it from another carrier (i.e. T-Mo, etc.), and if so is it subsidy unlocked?
=^..^= There must be a happy medium somewhere between being totally informed and blissfully unaware.