Neotel helping themselves to clients money

Neotel has billed me 3 times my monthly bill. I called the contact Centre and the consultant informed me that they will investigate and come back to me. What entitles Neotel to bill for services they have not rendered? My contract is setup to cut data at 100gb, the setup does not allow me to utilize more data, I have tried to purchase additional data before but due to their underdeveloped systems additional data purchases are not possible, I have had to purchase data on my Vodacom cellular contract to compensate for this dysfunctional service in earlier months. How dare Neotel just bill for unjust monies! Maybe looking for a self entitled bonus? Please check your Neotel debit orders are correct, if I can be over billed then so can you. Thank goodness my contract expires in February next month so that I can terminate it! Terrible billing service and below average service delivery.