Just don't bring your phone with you. With low power states and opaque software and hardware, you really can't risk it. You can never be sure it's truly off, unless it's in a Faraday bag. But is it worth it?
My phone has hardware kill switches for modem, WiFi/BT and camera/mic. So I can be sure. The schematics is available, too.
what phone?
Librem 5.
The magnetic field passes through a Faraday cage, so even then there are no guarantees if the phone uses unconventional modes of communication. Ultrasonic audio is another one.