Unilateral cost increases on signed contracts

I signed a contract with Vodacom in Decem I already received a notice that they want to increase the cost with 10%. That means Vodacom will increase the cost of these contracts twice during my contract period, making it more expensive than the contract I left. They get all the advantages of the contract while just increasing the cost unilaterally. This is not fair to consumers. They also did not tell me they can at anytime increase the contract cost.