Contact billing changed without legal documents sighed

Good Afternoon I have been signing or renewing a contract with vodacom for a couple of years. I do not understand how a legal signed contract can be increased during a 24 month period and is it legal? I checked my debit order on my bank account and see that it has increased by R28.00.So I phoned vodacom in the Midlands liberty mall and spoke to a lady by the name of Charmaine and enquired why is my installment increased and her reply was that Vodacom sent out smses and an email to notify customers of the increase. I asked her if it was legal and her reply was that we were notified. I did not receive any emails whatsoever to notify of changes to the contract. I know for a fact that is ******* to change a person's contract without signatures and paperwork done. I do not know what this forgery is being committed by vodacom and how much money they are making off every customer by charging this extra 28.00 rand. I told Charmaine I will report this on Hello Peter. com and she said I can go ahead and do it!!!!! As they are only an agency for vodacom can you believe it. I think customers should go and check their contracts and debit orders to the the forgery that is taking place. This company has now become rogues. I expect and answer to my query ASAP as I will have to seek legal help and advise