Can I retrieve an old prepaid At&t number?

If your prepaid number is still available, you will be able to insert that old SIM card and refill your account. It kind of depends on how long your Prepaid Plan has had a zero balance.  Prepaid accounts are closed and number lost if the balance has been zero for 60 days. 

I don't know who told you couldn't keep your phone number, that was incorrect. You can always migrate your phone number between AT&T prepaid and postpaid in a corporate store.

I suspect you were dealing with somebody in an authorized retailer store, or someone who wanted to get credit for adding a new postpaid line which does not happen when migrating a phone number between prepaid and postpaid

Hello @JessicaReed361, we'd love to provide you with more detailed information on how to switch your AT&T prepaid number to postpaid. 

In order to make the switch from prepaid to postpaid, keep the following details in mind:

• We'll need to check your credit.
• You can keep the same wireless number. If you do, we’ll credit any remaining balance on your AT&T prepaid account to your new wireless account.
• You won't get a refund for any monthly rate plan or package you already paid.
• We can’t refund unused AT&T prepaid cards.
• If you’re bringing your own device, it must be compatible with our network.
• If you’re getting a new device, it may have to be on an installment plan.
• A service-change fee may apply.
• You may have to pay a security deposit if you’re a business customer.

Also, in order to complete this process you'll need to visit the nearest AT&T store. If you still have your old number and SIM card, they'll be able to submit your request in store easily. 

We hope this helped and feel free to let us know if you have any other questions!

Thanks for contacting the AT&T Community Forums.

Lynn, AT&T Community Specialist