The two technologies ICE uses rely on permission for apps tonuse geolocation data for advertising purposes. Same reason you start seeing ads for local things when you travel.
Technically they can subpoena cell records to see which towers your phone connects to, but this is not viable for multiple people.
For privacy, if you care, having a rooted degoogled phone with no sim card is sufficient enough. You can check its signature by using your laptop as a IGW and capturing traffic to see if any apps or services ping anything.
If you want off grid comms, meshtastic devices are very nice, me and my wife use LilyGo Tdeck pluses for comms and finding each other at festivals. The portable modems are also nice because you can use them for GPS for your phone vs built in location services.