Transfer ATT home phone to Cellular plan

If this is a traditional landline the PIN in in the billing information. Read through this topic to determine the PIN. You don't work with AT&T you work with the carrier you're porting to, which you stated was T-Mobile. They will tell you if the number is portable and what they need from you. 

ATT said I need a transfer pin - I already know the account pin.  Will the account pin work?

Did you read the topic posted? Like I said you don't need to talk to AT&T and depending on the rep you talk to they may not know anyway.

The post mentions a 13 digit account number - My grandfather has a 12 digit account.

Should be the 10 digit phone number plus 3 digit account If this is a traditional landline. If it's digital service than yes you have to call for a PIN, I can't tell from your post which it is. If the phone plugs into the back of a modem it's digital.

It's a digital phone, but he uses it as a landline.   I know the account pin.  The att folks can't give me a transfer pin.   They said I have to do it in the online account but when I try to sign up for the online account they try to text an authorization pin to his landline/digital line.  He can't accept a text to his landline/digital line.  

Hate to tell you this, but you're going to have to work with AT&T. I have no idea why they seem to refuse to use email or a phone call to give you the verification information. You could file an FCC complaint.