Unfair practise

I have a 3 years contract with mtn. The contract is now in its 2nd year. Mtn increased my monthly premiums yet the service has deteriorated. Poor network coverage and whenever I check my balance I have to do it more than once as it always gives me the wrong information. I have been with mtn for more than 14 years & have never had any problems till now. Why increase premiums in the middle of a contract? I didn't sign up for that. I should be having fixed premiums. The least they can do is improve the services to match the premium increase