Is this a GSM plan/phone?  If so I would think you have a nationwide
plan one of the extremely old plans that is NOT to be offered but I've
seen some Rat Shacks offering these to customers. $29.99 for 250 day
and 1000 night and weekend mins.  They have this code in the computers
but it's not to be offered not to mention that the commission sucks on
anything that low.

The best way to check would be to log into www.cingular.com and set up
your account and see what is displayed there it will tell you exactly
what plan you have in a more plain english.