Billing system outdated

I received an sms from Cell C advising that my account is in arrears, so I checked my banking account only to note that Cell C did not debit my account on 2018/10/21 (debit date). I contacted Cell C and the agent tells me that Cell C did "go into" my account and the debit will only reflect in 2 days time. She further confirms that Cell C cannot debit on a Sunday (2018/10/21) as their systems does not allow. So because today is Monday Oct 22nd and Cell C did not process the debit order as agreed, my account is in arrears! In today's age, how can a company claim they cannot debit on a Sunday, then have the audacity to message me about arrears. on top of that i have to reply yes/ no to this sms??? Cell C... please update your billing systems!! I am not replying to your sms. I shudder to think how long I would have to wait for a refund should your system double debit me. Also, should your outdated system not run the debit if you do cut my services i shall complain to the ombudsman