Debt collector breaking law collecting paid up debts

I was called by these people and I told them that I have nothing to do with them as I don't owe MTN anything as they said. But the sent me the following sms:Banking details FNB ********** 0882 Code 250655 Use ref# ********** 1 & send proof to ********** Fax ********** 000 Bal R 7 113.83 on MTN acc I do not owe MTN as I have No contract with them the last time I dealt with MTN was in 2009 and I paid them off and moved to Vodacom, even so if I had anything to do with them that debt has already prescribed but in this case I do not owe MTN a dime yet these people just come out of nowhere to say i do after 8 years of me not being involved with MTN. I am disgusted and want them to back off my case they must check their records and leave honest people alone to live without being harassed. It's is ********* and ******* Regards Nomsa