Virgin Mobile debit order

I had a cell phone contract with Virgin Mobile it was a limited contract. I lost my job and don't have an income but I tried and make a plan and paid the contract amount out of my savings account via debit order. There were other debit orders going off as well but I was sure that I would find some kind of employment to make ends meet. Then the reality hits me, in my age group there is no vacancies specially in my type of qualifications. My savings were depleted and I started to make arrangements with all my deptors to make a smaller payment as not to cripple my credit record. this same arrangement were made with Virgin Mobile to a payment of R50.00/month. Every month they still request money from my savings account which has zero balance and the debit order gets send back as insufficient funds available. I then get penalized for an x amount. When I phoned the customer services at Virgin Mobile head office the person helping me claimed that the line was suspended and the only way I can stop the debit order is to settle the outstanding amount first. Now why did they accept the amount on which we agreed upon and they even supp**** me with a reference no. to pay via easy pay at Shoprite if they keep on requesting money from my savings account