Handed over after Terminating contract and number moved to prepaid.

I had a contract with MTN 2018 I quit my job; yet still paid the account when the contract ended in December i ensured to cancel the contract, the number was moved to prepaid & I used it for a while as a prepaid until it was recycled due to inactivity, now I get an email from SAYA recoveries saying I have been handed over to them, the contract I took on a Black Friday deal of 2016 so it ended 2018 Black friday I ensured to cancel the contract as I was nolonger employed. I went to the shop the contract shows it was fully paid for it even shows I had paid extra few cents. MTN is lacking a lot on service.