Poor security with Mtn

Went to apply for Mtn contract phone at Menlyn shop, when a consultant was busy with my application, he said that I've got another contact phone which I toke in 2016, he said this account has been handed to their Legal department. Tried to call legal department to find out why was I taken to legal department, they couldn't provide details as to why. I can't apply for contract phone because of this. I've been using my number for more than 12 year as a prepaid and to this date still and again when I ask as to how much I owe they said this account is not owing. How can I have an account that be taken to legal department when you not owing?