Frustrated customer!!!

I took a 24 month cellphone contract 3 years ago, when they called me to do an upgrade I told them that I don't want an upgrade, I'm canceling my contract. Yesterday I went into an MTN to take a new contract, I was turned away after being told that my account is in arreas and was handed to the lawyers. I called the customer service line and explained my frustration, I do not understand why I have outstanding fees, even if I had been owing, why did it not continue debuting from my account. This is in turn affecting my credit record I have an undisclosed amount in debt and I'm not even sure what it's for. I've never been called, messaged or emailed regarding this. Costumer service couldn't help me either and told me to wait for the legal team to contact me. Very frustrated and disappointed!!!!