Mtn over billing and cutting my services

In October 2020 I took a contract with mtn that was ending in October 2023, the contact was for 36 months, i called them toward the end of term to tell them that i will not be continuing with the contract but to keep my lines as Pay as you go. To my suprise MTN has billed me after the end date, my appeal to them to reconcile the account fell on deaf ears and instead they have closed the lines. MTN has overbilled me for 4 months and when I go to their branch they say they can not help because they are not authorized only head office can check peoples accounts further more when i call head office they just give me the runaround this has left me very angry and frustrated just to think ive supported this company since i first got a cell phone around 1999 and ive been loyal since whats really going on with mtn should we as South Africans just boycott you services because this is not what you would expect from a big service provider