Arrears amount dispute

I have been with CELL C for more than 10years and it was good to me but now,their service is unbelievable…To cut the story short ,My debit order didn’t go through for couple of months since last year November and nobody called,sms or emailed..if I didn’t call for the problem that I had with my Router I wud not have known about my BILL,now they telling I owe this huge amount I don’t understand how it came to that amount when I ask they tell me debit orders didn’t go through since 2021/01/30 and I have prove for last years until OCTOBER.WHY THEY DIDN'T BLOCK MY SIM IF I OWE THEM SO MUCH WHY THEY DIDN’T COMMUNICATION with me ,all they do now is email the case number and ask for my information.