account handed over

I had a contract with Cell c and on expiry, I decided to change to prepaid. My debit order date is the 25th of every month and Cell c was debiting my account on the 15th for 3 months. On the 15th April 2014 there were no funds in my account, the debit was returned. They tried on the 25th and it was successful. This was the last instalment. Cell c expects me to pay the R70 for the returned debit. I have received numerous calls from them and explained that they debited on the wrong date and I'm not liable for the charges. Today I received an sms that my account has been handed over to nds