Unauthorized Debit

In October 2017 someone claiming to be an MTN rep called at my workplace looking for me, at the end of that month I received a debit order from "MTN" of which I called my bank FNB and I stopped and reversed it. This happened again in November 2017 and December 2017 times three. I called FNB and they gave this number " ********** " and the name "Sandile Nsele"(sp), I have called countless times to no luck. This has negatively affected my credit record I've worked hard to restore and I can't get a vehicle today. I wish to find out how did this happen as I dob't even subscribe to MTN nor have a contract phone.