MBD have been harassing me for the last 2-3 years ...

MBD have been harassing me for the last 2-3 years on an account I have, on numerous occasions, informed them was fraudulently conducted by someone who I believe works for their client, MTN. They are trying to recover "outstanding" money from me based on activities conducted on my sim card that was removed from my phone. The SIM was removed and stored in my wallet because MTN kept increasing my billing limit, which I did not agree to or authorize and constantly instructed them to reduce. MBD advised I should take up the dispute with their client and not them (which I did and have been ignore ever since) and yet THEY keep calling me about the disputed amount and threaten me that I am liable for any calls they make to me in this regard! THIS IS OUTRAGEOUS!!!!! Harassment and threats - how different is this from gangsterism?! I feel Bullied.