unfair contract

I took a Mtn cell phone contract from Nasua Mobile in Pietermaritzburg on 2013. The recorded agremement was that Nasua Mobile will deduct its money on 15nth of every month and i will receive an airtime of R200 and sms's. When the airtime is finished i used to have an allowance of R20. That was fine with me and it was happening. Some time towards end 2014 i noticed that the Nasua is no longer deducting, Mtn is deductng and is deducting the money on 22nd of every month and there is no limit when tge airtime is finished. Now i receuved an sms saying i have out standing balance of plus or minus R450. I visited mtn branch in Pietermaritzburg but i did not get any help, I was told to phone mtn, i always phone mtn but i get no answer. I do not like this whole situation because i never had a contract with mtn on this phone at all. This phone no is ***, i request this to be investigated.