******* billing

I had a modem contract with mtn which expired in 2013 and I took time and effort to ensure that it is cancelled, then I took two cellphone lines with mtn again after that. The billing on the modem number *** stopped immediately, and was even removed from my billing statement, that's when I got rid of the reference number I had but surprisingly after two years they called me which was in November 2015 to tell me that they took me to their lawyers because I owe interest on the account. I asked why didn't they call me all along until now but couldn't get an answer but by luck the billing statement when the number was still active and after it was deactivated I have in my mail box. Please help because with the new lines they also made me pay over R4000 of data I didn't use but I paid it since I was afraid of being black listed because whether I'm right or wrong they will still do it, it does not cause them anything but me. I am currently on ITC because of this and funny enough I was never on ITC because I take care of my responsibilities.