Telkom is a *****

I changed banking details and Telkom was informed before the 10th March 2019 However on the 25th Telkom deducted on my old bank account which obviously didn't have any funds. The following day I made a manual payment to them and asked them to change my banking details accordingly. Now when I receive my Invoice for April I am being overcharged by R205 and when I query this with them they say it's a rejection fee. I mean why do they charge me when I informed them that my banking details have changed and someone didn't do their job? Also why such a big amount for a rejection fee? Telkom is being a *****. I can't wait for my contract to end with them this year.