Unknown contract with MTN

I have been with MTN over 18 years, and for the last 5 years plus been using the same phone and on, what I understood a month to month billing. Last week I wanted to upgrade to a new phone, and was informed by the MTN store that I need to cancel my existing contract direct by dialing 135. I was unaware of any existing contract, but contacted 135 only to find out that I now need to submit a affidavit, ID etc to the MTN ***** division for investigation. Also, every time I request information (over 3 days) on how long it will take and why I need to submit all this, I only get a reply to submit the documentation as requested and no further feedback. This is huge warning signs to me and I do not know if I should lawyer up or not. Big companies like MTN is quick to steamroller clients and therefore I am not willing to submit anything. All I want to do is cancel the "Unknown Existing Contract" and sign a new one. Don't know what is so difficult on this since I do not owe MTN any costs for any hardware etc. If so, inform me what the problem is and work with me so resolve.