Getting charged for a cancelled contract

I took out a new contract in Dec 2020 and cancelled in store within the cancellation period. They deactivated the number that was associated with the contract 2 or so months later, I didn't use it much so didn't care. They have now sent lawyers after me trying to claim for a contact that I cancelled, on a number that hasn't even been active since Feb 2021. No matter how much I try tell them I cancelled and have no active contracts, they are adamant I must pay. Its affecting my name and credit score now!