You tell them you had to buy a business ticket because there was no economy ticket available to purchase when you attempted to do so. They might ask if you have it in writing or a screenshot. You say that the booking was made over the phone, and that you were told economy is sold out, the cheapest available is so and so, and went ahead and booked it. Invite the insurance company to confirm this for themselves. They won’t do it because to prove that is next to impossible. They would need to know the time when you booked your ticket so that they can query with TG what availability was at that point. TG has better things to do and likely won’t do anything. Furthermore, all of this is moot as availability could have changed five minutes after your booking to allow economy to open up again.