Account settled, but still being harassed

Last year, I settled my account. It was confirmed as being cancelled as I could not even do a sim swop thereafter due to someone else using my number. The amount was paid at N1 city branch which every branch I go into confirms that the amount is on the system. The account was cancelled the previous year and that amount was apparently an arrear amount which I was not informed of at the time of cancellation. I was texted and emailed the amount that I needed to pay. That was in July 2014 that I paid it. Almost a year later, this year, ssda debt collectors are contacting me consistently that I am in arrears with a similar amount. What is going on with MTN? I need these people to stop harassing me. They want proof of payment, which the branches are not allowed to provide me with. N1 city branch emailed the legal department, but no response. Even though they confirmed with me on the phone last year that my account was settled and cancelled upon payment. I even complained on their website, but no response. This is unacceptable. I am ready to take this further, but I hope MTN will respond to this.