PROBLEM OF PAYMENTS

I HAVE A PROBLEM WITH VODACOM SINCE I HAVE CONTRACT WITH THEM I PAY EXTRA THEY SAY ITS BECAUSE I PAY LATE, SO FOR THE FIRST TIME I GAVE THEM MY SALARY ADVICE (PAYSLIP) THEY SAW THAT I GET PAID ON 15TH THEY DID DEBIT ORDER WHICH IT TAKES THE MONEY ONLY IN MONTH END AND WHEN THERE IS NO MONEY IN THE BANK, THEY PUT EXTRA HUNNDRED RAND ON MY PAYMENT SO CAN YOU PLEASE TELL THEM THAT THEY MUST CHANGE THEIR DEBIT ORDER TO 15TH, AND I CANT AFFORD TO PAY EXTRA AND FOR THE FIRST TIME WE AGREED THAT MY INTALLMENT IS R299.00 PER MONTH BUT NOW THEY INCREASE IT R318.00 CAN YOU ATLEAST ASK THEM WHAT GOING ON, BECAUSE WHEN I ASK AT THE SHOP THEY SAID I MUST CALL THE HEAD OOFICE I TRIED SO MANY TIMES NOW, ALWAYS RINGING. I WILL BE GLAD IF YOU CAN HELP ME I AGREED WITH R299.00 SO I WANT TO PAY R299.00 THANK YOU