Mtn cell phone contract

Took a cell phone contract,last year April contract came to an end,I paid up the last amount at their cellphone shop where i took my contract.i was given a slip as a proof of payment, then 2-3 months down the line I got messages that i owe MTN,send them the slip and bank statement that shows that i did made a payment to Mtn,but still they still send me text that i owe them,went to the shop that i made the payment and the lady who helped me made the payment was supposed as to what's going on Mtn,they've sent debt collectors who threatened to blacklist my name. I don't know what to do with Mtn now,I feel threatened now by Mtn and I'm not impressed with them and I would never advise anyone to take a contract with Mtn and their poor unprofessional service