Irregular billing from MTNZA

Irregular billing by MTNZA in my account is been happening for for some and I try several times to have fixed it seems they are stilling my money knowing that whatever they are billing me I'll have to pay I don't have a choice.Im so sick and tired with such treatment.Every month I payed a different amount and it's keep on going up .Just in February I pay my account twice cause they will block your service and there's nothing I can do.I just regret the day I sign up with them in that 2008.I try several times to seek clarity they tell you what they telling and you don't abide the service will be closed.