Over Charging & Suspending of Account

I used to be with Nashua Mobile. When NM closed down they handed all there MTN clients over to MTN. This is when my nightmare started. In the first month they suspended my service for non payment. 2 Days latter they switched me back on. SMS threats every month for non payment there after. I kept paying what i signed for irrespective of what they said. Was ready to close my account with them come December. I went in to MTN at Hemingways East London and asked one of there employee's if i was behind in my account. She checked and responded no. She then went on to say that i was due for an upgrade 2 months ago. I then asked her if i take an upgrade will they not charge me for the upgrade and my old contract. She responded no. I went ahead with the upgrade for an S6 for R429 per month. I paid what needed to be paid and asked once more if i am up to date. She responded yes you owe nothing. I got home to be suspended once more for non payment. Buy now i just ignored it. By Monday my phone was back on with no apology. Just an sms saying i have been reconnected. Now to pay my second month bill i get told i must pay R1100.68. I went ahead and paid my R429. I am now suspended once more. ???