lines closed

im on mtn contract with 2 numbers. *** for my phone @ R289 and *** for my modem @ R119. in april this year i received a billing of R3500 for my modem though according to my understanding and for the past year i have always paid a fixed amount of R119 monthly. when i sign the contract i was assured that my insatlment would be fixed @ R119. i asked several times to speak to the legal department but was denied the opportunity. MTN has now closed both lines because of this and im unable to receive calls. why do they have to switch both lines whereas i receive different billings for both of these numbers.... why is mtn misleading and taking advantage of its customers.