Vodacom does not stick to the agreed debit order date!

I have been with Vodacom for a very long time and I have never missed a single debit date for the payment of my account with them. My debit order date has always been on the 15th of every month, but recently Vodacom just decided to debit on the last day of May without any communication. My monthly payment is more than R2000 and I have never missed a payment. I recently upgraded my device and I was told that there would be a once off payment of a little over R135. I specifically asked the sales agent who was assisting me on the phone if that R135 will be added to my monthly bill on the 15th an he said 'yes'. To my surprise on the month end of May 2026 Vodacom tried debiting that amount from my account and there was obviously no money in my account. I was charged expensive bank fees for a returned debit by my bank, for something that is not my fault, all because Vodacom decided to debit money on any day other than the agreed date. I immediately called their upgrade department trying to fix this issue and they transferred me to the billing departments, the lady I spoke to in the billing departments wanted to transfer me back to the upgrades department and I requested that the relevant department gives me a call back because I was tired of being given the run around and it sounded like they were unsure of how to handle this. It has been almost a week later and no one from Vodacom has called me back regarding this, yet they keep sending messages that my account is in arrears. This is frustrating because it will affect my credit score negatively and if I am able to pay more than R2000 monthly without fail why on Earth would I fail to pay a mere R135?!