MTN Blacklisted my phone for no reason!

Last year my phone stopped picking up signal. I thought something was wrong with the device, but when I checked using a vodacom sim, It worked. I recently sent the device to Samsung centre for repairs. I was informed that my phone had been blacklisted and I need to contact my service provider. I was given a form to fill out and they need other documents like an affidavit. But my phone phone was NEVER LOST. I NEVER BLACKLISTED IT.<br> Why am I now going through so much trouble when I am paying? I am also now eligable for an upgrade. Which I wont be taking because, I am getting the worst service!