MBD Attorney - SHOCKING SERVICE

My name is Mitchell Ritchie and I have been harassed by the above company, they state that I owe them R12 795.17 for a MTN account, this was paid in 2017. I have sent them POP every single day, and when I phone them I just get told to email them. The letter they sent me stating who to email has the incorrect email address (that is what the debt collector told me). This is ruining my name and I will be blacklisted for something has has been paid years ago, surely this isn't legal. Surely MBD Attorneys could do the right thing and at least respond to my emails.