Too much billings, I don't even understand

I am currently having cellphone contract with them, Samsung s4 paying 330 and recently changed my account details then went to update them in July 2015 but they never deducted I then asked for their banking details then send it through absa bank, the problem I have with them is last month they told me I owe about R750 I didn't understand they then told me it's because I purchased data bundle of which I never but instead I paid R350 after Waiting For Them To deduct which the never Did Now they telling me I owe them R658 because there is a month I skipped.... I have never skipped I have all the slips I have been paying for all the months and I am on maternity leave I won't have that kind of money to pay them I have all the proof they need, I went to their shop to ask why are they not deducting they tell me every month that they gonna deduct on the 25th but they never. Please help me so they can deduct the money we agreed about because they are giving me a lot of stress now and when I call they all tell me different stories 1 would say I skipped, the other would say I purchased bundles. Thank you.... their response will be much appreciated.iam really sick with Telkom.