MTN and NuDebt is up to no good

My son has an account with them for his tablet. At a stage last year, he was unable to pay. In June, the arrangement was made to pay R800/month - the account was +/-R14000. We immediately paid that same month and ever since. The same month, they listed him on ITC. Recently, we have been called by New (NU) Debt Management. They handed him over. Neither MTN nor NuDebt is taking calls - either the line is busy or not working. ***/+***8/+***8. We have been sending emails to MTN's Debt department, the complaints centre, and to NuDebt, with all the proof of payments, and still nobody is answering our emails or adjusting the account. He is still owing R13000+ and recently R11000+. Nobody is listening to or helping him. They phone him and tell him they are proceeding against him, but they are not adjusting the account with the +/-R8000 payments made already. Don't know what to do anymore.