Paid off account - not reflecting - still getting demands that I must pay installment

A month ago I paid off my account in full. Now I get an sms that I still owe money when I know I don't. They even sent me an em ail to confirm that they received the payment!

I tried calling but no one ever answers. In fact, the phone gets put down on me.

I want this sorted out before I get blacklisted unnecessarily. I will involve my lawyer if you do not sort this out NOW! This isn't the first time I've had similar issues with you.