ATT did not record my payment, a confirmed electronic transfer

I use electronic bill payment at my bank, and it uses electronic transfers whenever the merchant has that capability.   ATT accepts electronic transfers; my bank made the payment and it is recorded with a confirmation number.

This month my bill shows that payment as missing.

Where on the MyATT web system can I post a billing dispute?  I'd like to file a challenge with attached files supporting my payment.

Thanks

Ray