Finding I have outstanding money a year later

Good day. <br> <br> I had 2 phones in 2*** period one my personal phone and one on mtn contract for work. The contract was for 2 years and started in may 2012 and I ended it in may 2014 as I had stopped working for the company. I thought everything was fine and I had made sure I had payed all the money as it was on debit order. So I stopped using the mtn phone and only used my personnel one since. I went into my current phones provider to do an upgrade and was told I have outstanding money on another account and it had been handed to mtn's legal department. I had no idea what they where talking about and tried to phone thought to mtn's help line but was cut off several times. And I still can't get a number to contact legal department directly. I now have bad credit on my name but was never informed or sent messages about this or even emailed with regards to it. I would like to know where the money is owed and what I need to do to get this removed from my name as I do not want a bad credit rating. And I still have been trying to contact anyone in mtn about this. The guys at mtn shop in walker drive helped me a lot but even they could not get far with regards to finding out this info.