MTN just decided after years that I owe them money, and without even letting me know, just put it on my credit record

MTN just decided after years that I owe them money, and without even letting me know, just put it on my credit record. I am calling relentlessly trying to get hold of someone to pay the money but after constantly holding I get transferred from person to person or told someone will call me back to just never be called back. I send emails and don’t get any response at all.