You're told it's not possible but that's, of course, a complete and total lie.
Of course it's possitble, ATT won't do it because they would lose a ton of cash in the form of overages.
Frankly, it's fraud: it's impossible for you to use the minutes you've paid for - either you go over and pay for more you can't use, or you leave yourself short. Either way, ATT wins.