I got one one off amazon that seems to mostly do what it claims. If I put my phone int the back it won't receive anything. If I really really want it quiet I put it on airplane mode, turn it off, and put it in the bag.

I second @chickentendrils@hexbear.net on printing your maps, learning to read maps, scouting out the area using google maps (preferably from behind a VPN). Also - Download and print out bus schedules for the area, and carry cash for a taxi if you can afford it. Having options is always nice if shit hits the fan, or just if some mundane bs happens like your car gets towed or whatever.

If you can afford a burner phone it's a good idea to have a burner. If you can manage without a phone it's worth considering. I don't think there's any up to date information on what police information warfare capabilities are in 2023. We used to be worried about Stingray devices that would act as a fake cellular tower to record identifying information for everyone in the area. Idk if that's still the case or if they've got worse shit now. I'm not 100% on this part; I've heard some phones, when they're turned "off", are actually in a low power mode and may still be capable of sending or receiving some information. idk, it's something I'd have to look up again.

Make sure to test your faraday bag before hand - Have someone call it or send yourself emails or something while vibrate is turned on. The bag I have has a roll top and only works if the top is correctly rolled down. Try not to use a DIY bag unless you really understand what you're doing. Apparently radio can slip out of even small gaps in the bag if it's not made correctly out of the right materials. Remember to keep all the radios - cellular, wifi, and bluetooth, turned off if you're not using them. There are ways to log identifying information from your phone if your wifi is on; The phone will query nearby wifi routers. (I think. Double check this, i haven't read up on wardriving in years)

Assume that your phone will be stolen by the cops and cracked; Log out of everything, back up your password app and delete it from your phone, remove all your photos, back up your contacts and only keep contacts you explicitly need right now on the phone, log out of everything, log out of everything. It's a lot of work, not everyone is going to do it, but ideally the only thing on your phone when the police grab it should be the phone numbers for hte national lawyers guild, your personal lawyer if you have one, the bail support network in your city, and which ever friend you're checking in with so they know you're alive, and essential apps like google maps. Signal was the preferred secure password protected encrypted messaging app for a long time but idk if we're still using it. On paper it's password protected and encrypted so the police can't read your messages or your call logs as long as Signal is locked, and afaik, in the US at least, they cannot legally make you unlock it without a signed warrant.

Also - make sure your phone is using a password. Not that facial recognition unlock, or fingerprint unlock, or anything else. They can hold the phone up to your face to unlock it. Idk if it's legal but they can force you to unlock it via the fingerprint scanner.

If they do get in to your phone, assume they created a copy of everything on it to hold as evidence, and too feed in to the US counter-intelligence databases. idk if they'd actually bother, but keep it in mind as a possibility; You're not just protecting yourself by keeping information off your phone if it's seized, your also protecting your comrades and community.

None of this will necessarily stop them from getting in to your shit, but you shouldn't make it easy for them; Make them use up time and manpower, make the city bleed cash paying for their overtime, make them drag a judge out to get a warrant for your shit.

Cops usually give your shit back eventually, but when you're weighing whether to bring your phone keep in mind that the cops might steal it or break it, and what that would cost you $$$.

I'm not saying you should put pigpoopballs as your phone background, but you could do that.

Disclaimer; I am borderline paranoid and read way too many spy novels.

Print a map! You can usually also disable data and cellular signals on the device and continue using GPS, with offline maps (Open Street Maps or Google Maps though I'd be wary of the latter's potential to store and sync location data once back online).

Also worth remembering bags aren't foolproof. Can rip, tear, develop small holes.

It doesn't exactly answer your question, but there are a few phones with hardware switches that will physically disconnect power to the modem and all the other radio components. They aren't exactly amazing, as far as phones go, though.

https://www.pine64.org/pinephone/
https://www.pine64.org/pinephonepro/
https://puri.sm/products/librem-5/

If they can grab your location metadata, they can hone in on when your phone disappears and then reappears to the towers.

If you're concerned about it, leave your phone at home. If you really need to have it on the trip, maybe park in a hospital or touristy place and leave it in the car.