Suspended Sim Card

I have a contract with Mobilesplus for about 3 years now. I have always been paying R35 per month in return for R55 airtime. They have always been debiting my account but about 3 weeks ago, they suspended my sim card. As an MTN subscriber, I went to MTN Nelspruit (Absa Square) to enquire and a consultant by the name of Nhlanhla assisted me by sending an e-mail to RMCS and they responded by saying that the dealer will call me, of which they did not. Again, I went to Absa Square and the same consultant assisted by calling *** and I was told that I need to make a payment of R312.00 which they say it's for 3 months' arrears for last year (2015). I was never notified of the outstanding payments and the only message I got was for a sim swap, which I declined. As far as I know, my account with them is up to date. This is the worst service I have ever received from any company and I am very dissasistfied with Mobilesplus and I want to terminate my contract with them. Please assist. My contact number is ***. Thanks