edward snowden himself demonstrated a phone case about 13 years ago that showed that even in airplane mode and even when turned off most phones emit radio signals welcome to 13 years ago, it's way way way worse now

this is true of google android and iOs, however I understand it is NOT the case for graphene OS

The only way is to remove the battery, which was easy on cell phones 20 years ago. Now the quickest and easiest way would be to destroy the phone