Account problem

I had a contract with incredible connection. I emailed their finance department requesting a settlement amount. after being told the amount I went to the store and paid the amount given to me in my settlement letter.

I emailed the following working day with the proof of payment and requested a paid up letter. upon receiving the paid up letter I was also told there was an amount that needed to be refunded to me and that I need to submit documents.

the following month I start receiving calls that I owe them money despite having paid it off completely