Legal action

I had the unfortunate issue of not being able to pay my account for 4 months and at the beginning of this month I tried contacting MTN to pay what was owed. I phoned the call center multiple times and after waiting 30 mins each time my call was dropped. I sent three emails to customer services and had no response. I went into the branch in Fourways and they could not assist. I even phoned MTN head office direct and asked for legal but was dropped each time. I eventually decided to take the amount on my most current Statement and paid the amount required of R6831 and an additional amount up to R8000 total. A day later I receive a email from lawyer claiming I owe MTN R18000. I immediately phoned them and asked why the amount was so high and they said there was over R11000 in legal fees for the few days they tried to contact me. I asked them on which numbers they had tried calling me to which they gave me the numbers that MTN had suspended. I asked them why they were only now emailing me and they said they had not thought of it till now. I asked for an invoice break down of the costs and they refused. How do I resolve this? Must I lodge a complaint with ICASA?