DMC STOP Taking Over Accounts that Are Paid up

My mom and I have been harassed by the DMC. They tried to takeover an account that I had paid up from MTN and tried to **** me. And when it all failed they started calling my mother as she was my next of kin on the contract busy telling her I should pay up. I sent paid up letter and they paused and now they are back still doing the same thing. I'll end up suing them for this kind of harassment because my mom is old and panics easily when people call for such reasons.