Very bad service ?

My mom took contract with MTN in 2019 and returned in within time. Now she's getting calls about being black listed. At first we were told the matter has been transferred to NMOYAKA Brooksluyt attorneys and we sent the email to them to try and handle the matter but we didn't get a response. At the moment she being asked to take out a fee for something she returned on time so that her name can be clear, which makes no sense to us.

Jeffery Mampa is person who sign off the return slip and the account holder is Mirriam Mampa