ATT Trying to charge for bill already paid !!!!! Ohio

Had ATT for land line, cell and internet.  Had problem with them in April and then same thing in June. Went to cable for phone and internet. Kept cell until it got shut off. Was sending payments on line to same place for cell. Didn't notice different address for them. They are trying to make me pay again for two months that were payed. Talked to customer service about four times. Finally found out payments weren't transfered to ATT mobile, which they were when I had land line.

Now have collection agency calling many times a day. Can't convince them bills payed .