Use a faraday pouch

Then you can't receive calls?

i mean proprietary baseband blobs already provide a back door , but does anyone have a reference for the perpetual ble broadcast ?

""" If the battery runs out or device is off

For supported devices, which include Pixel 8, Pixel 9, and Pixel 10 series, the Find Hub network can locate your phone for several hours even if the battery runs out or the device is powered off. """

https://support.google.com/android/answer/3265955?hl=en#zipp...

Couldn't find any official documentation from Apple, but there's this:

https://www.howtogeek.com/805624/what-does-iphone-findable-a...

Yeah, but proprietary baseband blobs don't control power, right? So if I turn the phone off, the radio ought to be dead... fingers crossed.

Not entirely. An iPhone can be "located" with it's find-my-phone features even if powered off (on by default, I believe you can disable this feature). Even if you disable it, you are trusting that it isn't discoverable in some form.