Cancel Contract for Stolen Phone

I got a contract out 2 years ago to help out a so called friend and he stole the phone. I therefore put a block on the phone and have been paying for it ever since. The contract is now up so I cancelled the direct debit. I tried to use live chat but can never get through. I don’t have the phone so sending a code to the number doesn’t help. Now a debt agency has contacted me I’ve rang them as you can actually get in touch with them but they say I have to cancel the contract myself. Someone please help this has been costing me money for the last 2 years for nothing, it has to stop. My stress level is through the roof.