Unhappy client of MTN

Sins when do you need to pay of a phone when you want to upgrade this is ridiculous. They expect me to pay R2200 for the phone i have to upgrade to new phone and then that is not all they put up my contract in the 24 months 3 times i started with account of R650 and now its currently on R770 this is not fair as we signed a contract for R650 for 24 months. How is this legal to do this to clients.