Vodacom increasing the price of the contract

When I bought my contract I don’t remember signing that you guys as a brand can change the prices without even letting me know which wouldn’t change the anger and disappointment I have with brand . The amount that I agreed and signed to pay should not increase because I agreed and choose a deal that I know I would afford If that’s how Vodacom does it’s business I guess next time I must choose a different service provider that will be faithful to the agreement that we both agree too.