Wrongfully billed and change of contract terms

Hi , I had a contract with Nausha Mobile and they have closed down , automatically the contract moved over to Mtn without my knowlegde , Mtn has now changed the contract which was a Topup R200 a month to an open line without my consent , i am now being over charged for something that i did not sign for . when i contacted Mtn they said they will investigate but nothing has happened , instead they are doubling the debit from my account .<br> <br> please help !! <br> your sincerly<br> Mzokhona Ngcobo