Seriously, if you have an opsec reason to need a phone to not have any signal 100% for sure... Put it in a microwave.

Do not turn the microwave on. Just put it in there and close the door. Microwaves are common and readily accessible Faraday cages.

There are plenty of reasons to have a secret phone and plenty of reasons to want no one to ever hear it ring. Now you know one more tool to make that happen.