Breach of contract

I would like to know if it is actually legal that vodacom can increase your cell phone contract after just about 4 or 5 months after you sign a 2 year contract with them, for example you sign a 2 year contract at 300 a month but then after 3 or 4 months they send you a message to tell you that you monthly payment will increase on the 1 of April 2024, if you sign a contract on the phone and what ever deal with the phone for 300 is it not supposed to stay 300 as you sign for 300 per month for 24 months. As far as I am concern this is a breach of the contract we agreed and signed, it is the second time ever since I took out a cell phone contact with them. What if you can not afford the increase because when you do take out a contract you go according to what is affordable but then they do this, to me I think this is ******* but maybe someone can explain to me so that everyone can understand.