Of course you shouldn't be paying it. You shouldn't have been paying it while you had the T-Mobile contract.
The only reason you had it was that the salesperson in the phone shop was on commission and possibly because you were given a cashback incentive for taking out insurance.
As you're not under contract, if you lose your phone you can either just buy a cheap pay-as-you-go phone or take out a new contract and get a new free phone.
Also, you may find that it's covered by your household insurance.