No Explanation to Suspect *****

I ported out of MTN to Vodacom in December 2012. Yesterday I received notice of a debit on my account from MTN. After a 1 hour wait on their call centre - it emerged that MTN has allowed someone else to activate my cell number on their network. Is this legal? Why were proper security checks not done?<br> MTN please answer me as to how someone else with just an e-mail address is allowed to activate my cell number on your network, and therefore have my account debited??<br> If this person is doing this to clone my cell number to conduct ***** - why are your security processes allowing this?<br> Furthermore - how can you allow the same cellphone number on 2 networks?<br> Your call centre person wasn't bothered to explain this to me - simply stating she will deactivate the number. I have a reference number for the call.