Flagged for *****....but by whom?

Whenever I've tried to apply by MTN I get declined and I get told my ID has been flagged for *****. The sales lady told me to contact TransUnion directly as it's not from MTNs side. I have contacted trans union as well as SA ***** and my ID isn't flagged. I've been trying to get hold of the ***** dept at mtn for hours to no avail. All I want to do is find out where this flag has come from as its concerning my ID has been flagged for *****. Its not from TransUnion like I was told so it must be from mtn itself and I don't know why. There seems to be no one to speak to either.