So I have a Samsung phone that is no longer being used as it has been replaced by my Pixel running GOS. I want to turn this phone into a completely offline music player.
It has no SIM and won’t connect to Wifi. My assumption is turning on Airplane mode will be enough.
Is there anything I should be concerned about if I do this?
If you’re going to use it as a music player with only mp3-files (I’m presuming) on it and nothing else and connect it with AUX(?) to speakers/headphones, then I don’t think it really matters if it’s perfectly secure or not.
It will still ping cell towers every time you boot the device up.
Turn off WiFi, Bluetooth and more like that and turn on Airplane mode, if it has a headphone jack also even better, you can also use USB File transfer to transfer your local music, videos etc.
Honestly nothing to be concerned of if you’re doing above but hey feel free to correct me if I miss or said anything wrong
Does Android actually turn off Airplane Mode on reboot?
Ugh that’s annoying yeah
If it’s a modern enough one, maybe you could sell it and buy a real dumb mp3 with a better battery life that doesn’t require any hacks? 
Not sure how you like your music’s bitrate or if compressed mp3 is just fine with you. 
I think what you say about this is also applicable with most mobile phones with no SIM attached.
I am getting a Hiby Digital Audio Player and while it is Android, it does not have a GSM module so it does not ping to cell sites on boot. Maybe consider getting a dedicated DAP as well?
It does not.
Airplane Mode correctly persists across reboots.
But it may be possible that the modem scans for towers before Android actually boots and invokes airplane mode.
I see. Thanks for letting me know!
OK, kind of a wild idea here, but cheaper - get or make a cheap Faraday bag and when you start up the phone, put it in there during startup. Give it 2 minutes or so. Then with the phone in the bag, turn on airplane mode before you pull it out.
The problem is most I found online have Bluetooth or some other wireless radio. I don’t want that. I can plug in wired headphones into my old phone.
The only other option I can think of is an iPod which are very expensive used.
The phone is a Samsung A15. It is a very budget phone. I doubt it would sell for much.
Is there anyway to mitigate this in Android?
Not in a consistent and safe way IMO.
Besides a faraday bag on boot maybe.
Even without SIM card (as mentioned above) ?!
I forgot the details on that one but this is a nice episode about that topic
https://odysee.com/@NaomiBrockwell:4/Calyx:7
I think these elements must be considered to assess the ability of a phone to independently connect to a cell tower :
Concerning the Google Pixel phones, well-known on this forum :
Modem could ping cell towers with or without SIM cards. SIM card is like the key of a door lock, modem could knock random doors with or without a key. If it is out of emergency (like 911 /999), the good souls behind doors will engage even modem does not have a key.
The behaviour is down on to the baseband firmware.
Information about baseband firmware is very limited due to its closed-source nature, and there are many baseband processors in the market
Some studies suggests that the claim is not evident (for the tested models with that firmware version at that time). However, it is just a snapshot, and it could simply because the authority or manufacturer did not “activate” that feature by the time of testing.
I sincerely don’t think it is a concern unless you are the most wanted person in a country. Some of my friends, who have high threat levels, have quite crappy OPSEC and are still fine after being hunted by an authoritarian regime for years.
Of course if the adversaries want you enough, they could pour millions to hunt you down, exploiting whatever zeros days they have. But you should understand your threats and threat level before panicking or overreacting.
For 99.99% of the population, there is no need to go for an extreme route to remove and hide all possible tracks.