Telkom contract are the worst they rip people off their hard earned money

I had contract with Telkom fir 2 years after completion I cancelled for 3 months I was only paying my device insurance then later in the forth month they billed me for a cancelled device on asking they say it’s because I never told them to cancel the device while the contract was completed, my question was if I didn’t cancel why the other 3 months I was not but only billed for insurance their response was they don’t know but if I don’t pay they blacklist me and I should not ask questions but just pay. What kind of madness