All prepaid phones are limited to 1 month. However if you look at the ATT prepaid options it will also give you the option of putting 100 on your prepaid phone. This balance will expire 12 months or once you have used it.
If you chose a contracted line then they have a way to bill you for your usage thereby keeping your phone number active. Since you have a prepaid phone you have to pay in order to keep that number active each month or pay more and get an extended expiration date.