contract not cancelled

Worst network ever...horrible service and very expensive I have been a loyal customer for decades , took a contract for 24 months ,that ended in March/April 2020 which was fully paid I received a call after few weeks to advise me I'm owing because I still used their airtime for that month I paid what was owed after after 6months I received a call that I owe them almost R700 really how for what....???? coz I asked them please cancel my contract after the first incident all calls are recorded please refer back to that i will not be liable for any fees accumulated coz my account was not closed