Billed for a dishonoured debit order

I was a customer of Nashua Mobile for +-30 months, until +-4 months ago when Vodacom took over my cell phone contract. I started my contract with my Nashua Mobile contract being debited on the last day of each month. Well when Vodacom took over, the 1st month, I was informed of my debit order being dishonoured, due to insufficient funds in my bank account, I was told to pay my account immediately, or my line will be suspended. I could not understand, because I don't draw any money, until all my debit orders are deducted. I had to ran to the bank, draw the money, and pay it into Vodacoms account. I contacted them on numerous occassions and emails......which cost me more than R100.00 already. They have finally changed my debit order payment date to 1st of the month...but my complaint is Vodacom said when the debit went off last month (26 January 2015), there was no funds in my account...that was supposed to be 31 January 2015, The 5th February 2015, the debit order was debited.....still my money and still sufficient money to pay Vodacom...now my problem I have is, that Vodacom is harrassing me during my work for arrears of R100.00 for a dishonoured