Totally unacceptable

In December someone in the mtn organisation did a sim swop on my sim. I called straight away and was told to hold into a branch. Which I did. I was assured this was an admin error and that my account was still in tact. Fast forward to January my debit order comes through as R17k. Which my bank bounces thankfully. I then call mtn and get told this will be sorted out. In February again I see that debit order waiting. I complain again and get assurance this time it will be sorted. Now this month's debit order the bank allowed it to run and now I'm in overdraft. How can this still be going on when I have spoken to managers and team leaders. I still have no correspondence from the ***** department. And im sorry why must I pay for a mtn employee who has **********ly copied my sim?