Vodacom

I have had a Vodacom account to the value of R1400 per month for over 10 years. Recently R100 was added to my contract with no additional service requested. After three visits to Vodacom I was told that due to the increase in interest rates I was told by the accounts department that Vodacom has the authority to add 5% to all accounts due to the Reserve bank interest rate increases. Does Vodacom have the authority to do this when in fact I am not loaning money?