Unhappy with Vodacom Service

I have a fixed wifi contract with Vodacom to the value of R129 monthly. It is only after my phone contract was cancelled, did I notice that my wifi contract is billed at R139 monthly. I called the contracts call centre who explained that the subscription was increased by R10. I then asked for the proof that I agreed to this increase - to which the call centre agent was not able to respond to. I then asked that a Vodacom manager contact me and have not received a phone call yet. Vodacom will have to do one of the following (1) reduce my subscription to R129 and credit me for the months that I was charged R139 instead of R129 or (2) Release me from this contract now and on Vodacom's own costs, as I did not sign up for a contract of R139 per month. I am in the process of checking on the National Credit Act to see if this increase is even legal.