Having worked for two major networks, the insurance is fair, you need to uphold your end of the bargain.
Reporting the phone lost/stolen is the
first thing you should do, as
@byegad will agree with me here, it means the SIM card can not be used to make calls/texts etc at your expense, the phone and SIM are barred from the network, and then, in the instance you do find it (which fortunately I had a customer who did), the bar can be lifted without any costs to you.
Ok, it may not be on the top of your list of priorities, but it means you don't get a shocking bill at the end of the month, claiming for calls you never made. I'd rather have the inconvenience of making two calls than one about a high bill.