I have a phone that has no sim card and I only use it for podcasts (using wifi). I found it strange that different places I took it changed how long the battery lasted. I was going on the idea that it was about the wifi. But as I traveled to different places, and different countries, I saw a radical change in the battery life. I could go over a week in small country and then in the US it would drain in two days.

I finally turned the airplane mode on and it returned to the level of outside US small country in terms of battery life.

What the hell is that device doing when in the US? Its terrifying actually.

Leave your phone at your alibi location.

In some countries you still can call emergency services without SIM cards, so your phone can still reach the network for this purpose

Radio refers to the network, so yes for it to not connect only airplane mode works in my knowledge (you can activate WiFi after)

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Short answer: Yes The network can see your IMEI and triangulate your location even though no SIM is installed. and theoretically with airplane mode off no radio is on

The radios are the parts of your phone that communicate wirelessly. Most phones will only turn the cell radio off entirely during airplane mode, disabling mobile data does not typically turn that radio off.

Airplane mode should turn everything off (unless you re-enable things like WiFi, but that should still keep the cell radio off)

Yes, the IMEI uniquly identifies the phone itself, so if the GSM radio is on, the network can monitor it’s approximate location.

There’s a couple of caveats: IMEI cloning is possible, but unlikely, and he accuracy of the triangulation will depend on a lot of factors including how far apart the towers are and what sort of obstructions there are between you and the towers. My understanding is that it is done by comparing your signal strength at each of the towers as a proxy for distance. If there’s a large obstruction that reduces your signal to a tower it could throw those measurements off. They’ll know you’re in the area, but not exactly where.

Radio here refers to your mobile (non-wifi) wireless connection, though technically, Wi-Fi is also radio and can be used for triangulation purposes by access points. Technically speaking, any communications method that uses electromagnetic waves is radio, regardless of modulation and data encoding.

Does this mean our IMEI is identified by cell tower as long as the airplane mode is off?

The IMEI is how the cell tower differentiates the individual network participants. It can be compared to a MAC address.

Radio is referring to the proprietary piece of hardware that is in every cell phone that handles the physical portion of transmitting radio waves for wireless connectivity. This piece of hardware generally handles LTE/5G, wifi, and bluetooth.

Your IMEI is the identifier that is burned into that hardware, which uniquely lets the carriers determine if your device can authenticate on their networks.

You can turn off the radio by disabling your cellular network or turning on airplane mode on your phone. You will temporarily lose access to your cellular network until you turn it back on.

Newer devices don’t require SIM cards to connect. They have eSIM cards that can be used. You still need one or the other to connect to a cellular network.

Put simply the radio broadcasts a sort of hello message to the tower so the tower knows where to listen (this is about signal direction or beam shaping, but imagine the eye of Sauron swiveling to see Frodo). This includes the identifier of the handset, the IMEI number, so that the tower can keep track of who is who. The second step of getting connected to the network is done with the details inside the SIM card, specifically the IMSI number.

If your phone has no SIM card you can still make an emergency call. You can also have an eSIM which is a software version of the SIM card. In both cases you can bypass the SIM and get connected.

If you turn airplane mode on the radio is powered off in theory, but this is not absolutely guaranteed. It should be off, the system will report it is off, but there are fringe cases where it may still be very slightly active, usually from malware or similar things.

So no SIM means no IMSI, but the radio itself has the IMEI and that handset is hard coded to that identifier. If the radio powers on it will broadcast the IMEI to negotiate connection with or without the SIM and IMSI.