Bad service

I recently took out another contract with telkom I was told that my account would be debited on the 5th of December but they never did I have been trying to get through there call Centre for weeks but with no luck until I finally got through to someone on the 24th/02/2020 the guy was helpful but the query was not resolved he told that it was a mistake on their side that they did not debit on the 5th of December so they will run a double debit in January, I asked him how is it my fault that they did not debit, was also informed that am in credit with but they can not use that money towards that bill I just don't understand why is it my problem that you guys failed to debit in December and now want to run a double debit in January