MTN repair charge

A few months ago, I can't even remember the date I handed my cellphone in at a walk in service provider of MTN. My phone was faulty, I had problems so often with this phone. I never heard from them for weeks so went in to enquire only to be told it had never been sent off. They would do it immediately. I was told that it needed to go in for repairs. My phone was still within the 2 year contract and as far as I am aware MTN should either repair or replace. I received a call from their repair centre quoting me an amount of over R700. I said the phone was not anywhere near that as it would be coming up for renewal in Dec. I have received numerous SMS advising me of the following IMEI No *** Job card no *** saying reminder: service charge payment due of R330 incl Vat. If I do not pay in 2 days they will sell it to defray costs. Queries number *** which I called last week and they took my details down to have a manager call. Up to today, no one has called. I have been a loyal client of theirs for over 20 years but this is going to change as I no longer want to feel I have been taken for a ride and abused by MTN. Totally unhappy.