No customer service

I have a contract which I want to cancel. I was told to phone in April as the end date of the contract is 18 May 2021. I phone now and now they want to charge me a early cancelation fee because I must only phone on the 18th of April. I have been paying this contract and not using it. They have received money every month without anything being used. Now they cannot accommodate me to cancel the contract a few days earlier