VODACOM change contract with no customer consent

In March I was sent an sms saying that all Smart+ price plans on my account were to be increased and that the data allocation would be increased from 1 May. What gives Vodacom the right to increase contracts without the customer's consent? I called the Help Desk (bit of a misnomer, because there is nothing helpful about a help desk!) and they knew nothing of this SMS. They kept trying to tell me it was the increase in VAT, but it is not the VAT and in any case VAT increased in April.

After multiple frustrating calls I was told I could not log a call until the increase went through! I called in May as soon as I got the May invoice showing that there had been an increase in the VAT EXCLUSIVE amount. Now each time I call to object to this I get an SMS from Afsana Osman in Vodacom Business Support reminding me of the SMS sent in March !!

I do not want the additional data (which is being charged at an outrageous rate per MB) and I do not see why Vodacom should be allowed to change a contract. In the past I tried to have a contract changed and was told I had signed for 24 months and there was no chance of having it changed.

Imagine how much money Vodacom is generating if every contract has has this small increase app**** ... I am sure a lot of the unsuspecting South African public have not even noticed the increase on their account.

My reference number for this issue is A2-AVR4-18RXU