MTN is ruining my credit report

I had an account with them which was fully paid according to my knowledge. I also called a month before to cancel and they told me I should call 2 weeks before my contract expires. I did exactly that. So now 2 years down the line my credit is in default. Last time when they called me to tell me I'm owing them cancelation fee amount I requested an contract section where it states the cancelation fee since I wasn't told about prior. So I can fix the payment but till to date nothing was sent to me. I require MTN to take off the default under my credit record while we fix this.