INCREASED RATE, on a already signed contract

I Have signed a cellphone contract with Vodacom, last year December. Ive signed a contract with them for 2years. Now I got a notice via SMS that my contract rate will increase as from MAY. I then phoned customer care, the lady said she is not sure, according to her it is only on contract that is older than a year, I spoke to her supervisor, he said that Vodacom made some calculating mistakes, and this was his reason. This is legally not right.