Breach of Contract

We have phoned vodacom many times since December 2015 to correct the invoice on my number as they are over charging me for my contract. Whomever i speak to keeps saying its been updated and yet i am still being over charged. Should this not be resolved before the debit order now at the end of February 2016 the matter will be taken further.<br> <br> My bill is ment to be a set amount of R86.63 and i am being charged R139.90. I do not understand how it went well from June 2015 till November 2015 and then all of a sudden the contract price changed.