Unhappy customer

I upgraded my contract in July with MTN Westgate Mall in Mitchell's Plain. Since August I was billed for an unreasonable amount which has resulted in me being in arrears. I have contacted and visited the store where I took out my contract on numerous occasions and I still haven't received feedback as the consultant said she would contact me. My line has been blocked since then and it seems to me like they are not interested in assisting me with this query. It is frustrating as I can't use my phone. I have been with MTN for about 10 years and I have never experienced this before. Is there anyone that can assist me with a way forward so that I can make use of my phone. The consultant who so called assisted me is named Mercia and she works at the MTN store in Westgate Mall in Mitchell's Plain.