Call from attorneys for a Pay as You Go

Hi, I keep on receiving calls from some attorneys saying they calling on behalf of MTN for an outstanding account to the value of -+R 1500. I have a pay as you go with MTN and I don't owe any money to anybody. I can't call 173 as a recording says the number does not exist, if I call 808 and opt to speak to a service champion or what ever they are called a recording says I called too many time and should call again next month. I only called once to 808. How can I owe money on a pay as you go? These people are harassing me and putting my job in jeopardy. I was sleeping for a night shift when they called me today on my private, unlisted home number. This is getting ridiculous, how do I stop this harassment?