Debit order for account paid in advance

I took out a several contract with MTN last year, and I paid them all in advance. Earlier this month, I was debited around R8870, which was the exact amount I was in advance with on the account. I queried the debit, and I still haven't received any feedback, whatsoever. Ref no: ***.

Today, another amount was debited, around R10 000, this time. Now I am stuck with unnecessary charges on my Cheque account, to the amount of about R600, because both debit orders bounced, and the account was overdrawn. I mean... The account was paid in advance. How the hell does such a thing happen? I am Disgusted at how MTN has approached this whole situation.