Very bad service mtn sa

Someone used my identity to open up a contract at mtn, they gave him 5 hand set all were s 21, hw is that possible they said he paid up 10000 cash upfront, and whn i query that, they said am must open up a case of which idid and i sent them documents as they have asked, after a couple of i recieve a call frm dept collectors, and that came to my suprise because mtn after i sent them doc they never contacted me regading the case, only fone calls frm dept collectors, and that is so cruel because their neglegence cost me to go to credit bereau because i never have a contract with them, worse part they dnt tell me hw far with their investigating, i find my self helpless because what they ve asked me i did, i dnt knw wat to do nw