Vodacom ripping me off

I took out a cellphone contract with Vodacom in December 2015 on the Smart S option. My first month (January 2016) went off smoothly, my issue is as of February I do not understand why my bill has increased to more than double my premium which is R529 p/m including insurance of R250. My airtime and data is used within a few days of my new month subscription and then I am forced to contact Vodacom to open my line over and over again (I honestly feel Vodacom charges are exorbitant). In February 2016 I had left money in my account for my Vodacom debit order however it was returned due to insufficient funds but a few days prior to the debited order in contacted Vodacom to get my invoice balance that would be going off on 01/03/2016 and in left that amount to be deducted. Now I find myself in terrible arrears because I do not receive an invoice from Vodacom since the inception of my contract. I have entered into an arrangement with Vodacom and have paid R1100 towards the arrears. Vodacom refuses to accept that I will pay my monthly installment and R500 towards the arrears. I wouldn't be in this situation if Vodacom sent/sends me my invoice. Please assist me in rectifying this issue.