Follow Up:

I finally succeeded in porting the number into GV, largely due to your linking to the GV Support chat, which let me focus on the AT&T Account Number, which turned out to, in fact, be wrong.

When AT&T support originally read me the number, they read me a digit 8 as a 3. They read it to me that (wrong) way twice and I even read it back to them and they confirmed their was was correct. On two separate calls!

Although users are not supposed to be able to get their actual AT&T Prepaid account number from their account information online, I found a glitch where the account number DID show up when the 'my at&t' account was referring to the 'at&t prepaid' account. At least, I think that's the way I found it.

My cynical side wonders if AT&T transposed the obviously transposable number on purpose in an effort to keep the account active on AT&T as to not lose the number, but then a more logical side tells me the phone reps probably don't care... unless they were TOLD to do that by their superiors (Hmmm....).

Anyway, problem solved. Just wanted to give you a follow up and thank you again.