What is going on???

I have two contacts with Cell C, every month they debit my account with varying amounts on varying dates. Their explanation for this is that there are not enough funds in my account at the time of the debits being processed. However on numerous occasions I have phoned the call centre and asked them to change the dates of the debits and explain why they are charging me so much. This problem has been on going for the pass 6 months and every time I lay a complaint I NEVER get any feed back for Cell C! This is the worse service I have ever received and the best part is they will never admit they are wrong or refund any money that may be due to me. According to Cell C, the service provider is NEVER wrong and I can't wait to terminate my contacts with them!