The decent thing to do is to tell the Insurers. It really depends on how to told them it was lost. Finding it under a pillow after telling them you'd searched diligently for it and there was nowhere it could possibly be. 😀
Hopefully they'll respect your honesty and just tell you to keep the replacement. You'll need a new SIM to get the old one going but at least you get to keep your Contacts and any other personal files.