Phone harassment

I have 2 creditcard accounts being handled by MBD. My arrangement has been to pay R250 per month on each account, and it would be paid on or before the 15th of every month. Every month I get between 10 and 20 calls from more than 100 different phone numbers, to remind me to pay, or set up a debitorder. I have never missed a payment in more than 5 years. For more than 5 years I have been trying to tell the callers that I cannot put the payment on a debit order as I do not have a fixed date that my salary (commission) is being paid into my bank account, to no avail. Two weeks later they start calling again. Today, 18/12/2023, I have already received more than 10 calls, identified by "Truecaller", three days after I paid the accounts. Lately, as soon as I answer the phone, the caller puts the phone down without speaking. Is that just to confirm that I was called, and another amount of administration costs can be added to my account? I am 65 years old, and medically disabled, (heart function only 25%) but I keep on working to pay my accounts. I do not need all those phone calls (harassment) to artficially inflate my account. I do not know what to do next. Ombud?