Wrong amount being deducted every month

My cellphone and data contracts were at Nashua Mobile. When Nashua Mob closed all their branches in South Africa the end of 2014, all my contracts went to MTN directly. One of my contract amounts was wrongly captured, because since my contract was handed over to MTN, the amount is wrong. It is a 2GB data package for 24mnths. The contract amount should be R129.00 p/month, but from Feb2015 MTN deducts R389.00 p/m on debit order for this contract. I phoned MTN on 02March to enquire about the wrong amount on my account. I've send an email with the contract attached to MTN on 02March, but I had to send 2 more emails on 17March before I received a reply on my emails. MTN gave me the following reference: ***, Req:*** and Req:***. I received an email from MTN on 04April saying that the request were escalated to the billing department for assistance. MTN still deducts R389.00 per month. I contacted MTN several times and every time they say that the right amount will be deducted the end of the month, but the wrong amount was still deducted end of May. I want the right amount deducted p/m and I want a refund for the huge amount over paid and I want to know when it will be.