Bad service / experience I have receive from mtn RSA

MTN RSA I'm a customer of mtn for years but they have treated me so bad I had an contract with them which was a month to month contract then out of the blue they put me on another contract without my permission now they're saying I owe them for something that I didn't sign for. I've been asking them for a signed copy of my contract for years now but no response and now I'm receiving mails from lowers for the contact that I have never sign up for I'm worried that this going to affect my credit score in future because I am not gone pay for something that I didn't sign for