I Opened an Account I can not Access

I opened an account for my elderly father. When I opened the account, I was receiving physical bills to my mailing address and I managed to set up auto pay over the phone using the information on the bill.

I have since moved, and my credit card expired. I no longer have a bill, or receive any bills in the mail since my address has changed. I don't have an account number or a support pin(?) so the folks on AT&T's phone support couldn't help

I need three things:

1. To update the address on the account to my new home's address.

2. To update the auto-pay payment method (and pay any outstanding balance) so that he can use his phone again.

3.  Find a way to manage this account while I have no physical access to the home phone number on the account.

But I can't do that because I can't create an account (the portal prompts me telling me they will call  his phone and issue a code), can't get support over the phone because I don't have a support PIN(?) or the account number. I'm out of ideas.

Please advise,

-Chris