AT&T bill paid, but they say it isn't even though I have proof.

This is the first time I have had this problem with AT&T and I can tell you, I have spent 3-4 hours SO FAR trying to resolve it.  I have to go in again today to my local AT&T store, for the second time, and try to resolve the situation.  Here is how this has gone so far:

1)  AT&T notified me that my bill was paid prior to my scheduled electronic check was due to go out.  I logged into my account and saw that it was paid by a MasterCard (under payment method column) that I didn't own.  I checked my previous 12+ payments and the payment method was always electronic check sent by my bank, that I set up manually every month.  

2)  I called AT&T and let them know.  I told them my payment was coming in a day or two.  The lady said this happens and sometimes a payment gets paid on an incorrect account by a customer.  She reversed it.  (I will never do that again).

3)  I checked in a week later, once I saw my electronic check paid and it was removed from my checking.  They didn't show the payment yet.

4)  I waited longer and it still didn't show.  I went to my bank, got a printout proving the electronic check paid and that it wasn't returned.

5)  I called AT&T AGAIN.  They said the payment was reversed on the 19th when I called, I explained, AGAIN, why that was and she said they couldn't see any payments from my bank.  I told her I had proof.

6)  I was finally sent to an accounting department to which I had to go through the SAME EXPLANATION all over again.

7)  Frustrated, I asked to speak to his supervisor.  He kept asking me why I needed to, and I kept explaining why.  

😎  Finally, got to a supervisor - went through the WHOLE EXPLANATION again, to which he said I would need to go into the local AT&T store with my proof as they don't take faxes or emails - I would have to show it to the Rep, once he/she saw it, then they could apply it as the payment.

9)  Talked to the Rep in the store, he was great.  However, after an hour plus in the store and him placing two calls, telling them he had the proof, THEY TOLD HIM I NEEDED TO FAX THE DOCUMENTATION TO THEM.  THEY JUST TOLD ME IN THE PREVIOUS CALL I MADE, THAT WASN'T POSSIBLE!  

10)  So, I faxed it that same day.  The Rep told me they said to check back in 3-5 days, he said make it 3 days, so that is today.  He also said to come into the store and they would make the call again.

11)  So, today I am going in AGAIN, ON MY LUNCH HOUR so another Rep can call and argue the point - seeing as the REP has my documents and they were faxed as well.

12)  THIS HAS BEEN A NIGHTMARE AND IS RIDICULOUS.  I continue to get emails that my bill is past due and IT ISN'T!