Annoying and rude phone calls

My wife is frequently getting phone calls from SSDA asking to pay for an MTN account she doesn't have. She explained multiple times that she doesn't have or had any accounts with MTN, but it doesn't stop these rude calls from continuing. We checked at MTN for her ID number and there are no contracts/debt on her name. The MTN reference that was mentioned in one of SMSes is ********** 155 which is not helping. This is extremely annoying and unacceptably rude telephone behavior. Get your data in order an focus your energy on chasing people that really owe money.