MTN charge me for someone elses calls

I have been a Nashua mobile client for the last 18 years with a MTN number. Only received brilliant service from Nashua mobile. Since the closure of Nashua mobile around October 2014 my account was moved to MTN directly. What a disaster I have to say. FIRSTLY MTN must have the worst call centre in South Africa but thats not really my complaint. Since moving to MTN I have been paying thousands of rands for calls which I did not make. I FIRST notified MTN in late 2014 of the problem, only to be told by the call centre they can not see my latest account on the system and I have to email some accounts department for information. Never got response back from anyone at mtn. Last week I again notified them that I am being billed around R2000 a month for calls I am not making. Again I was told that they can not help me and they will forward my complaint to someone. To date no reply. I am still being billed for around R400 for calls into africa which I am not dialing. On top of that I am paying for hunderds of local callS notcoming from my sim card. I really do not know how to sort this out as MTN is just ignoring all my attempts to sort this out.