Before you go to the Ombudsman and waste your time, what reason did the travel insurance give for declining to pay out?
You need to carefully check your policy documents. Did the travel insurance actually include phone cover, did it cover theft?
If you paid for the bargain basement travel insurance that didn't include phone cover then of course they are not going to pay out and going to the Ombudsman will be pointless. To be blunt when it comes to tickets you seem to have bought the cheapest most restrictive version and expected all the flexibility of the more expensive option, so it wouldn't totally surprise me if you haven't actually got the cover you are claiming for.