I just discovered I can receive calls+texts and make calls with airplane mode enabled, does this mean identifiers like my IMSI, IMEI and geolcation are invisible to my cellular network?

What do you want advice about?
Wifi calling, IMSI, IMEI, cellular network

What have you considered or looked at already?
I’ve done some research about cellular networks, phone hardware and wireless transmission technology.

In brief, tell us about your privacy threat model?
I want to reveal as little information and data about myself and my activities as possible to as few people and organisations as possible, and make the acquisition of that information as difficult as possible.

Let me be as clear as possible with what I’m describing: I am using GrapheneOS, a Pixel 7, and I don’t currently have a SIM card inserted in the phone. I purchased it as anonymously as I could, with cash, downloaded android updates, installed GrapheneOS and enabled airplane mode while I was there. I never put a SIM card in the phone.

On a separate Android phone, which has a SIM card and a registered number, I enabled airplane mode and turned wifi on. Surprisingly, I’m not only able to make and receive calls with wifi calling, I can also receive texts. Note that I am NOT talking about Signal, I am talking about regular, unencrypted calls and texts without any special app.

I am fairly new to the technology of wifi calling. What confuses me is that you can use WiFi but with a cellular connection (eg. 4G or 5G), but you can also use Wifi calling using your home network provided by your ISP. Naturally, you can also use Wifi calling using public wifi. What confuses me further is that you can actually enable airplane mode, which (should) disable the cellular radio on the phone, but you can still make and receive calls to/from that number, and receive texts to that number. I’m wondering how that is possible. I’m thinking that normally, the SIM card sends instructions to the cell module, but when airplane mode is enabled, it communicates with the wifi module. The difference, as I see it, is that the IMSI and IMEI is not being communicated to the network, because you’re not using a cellular network. You’re using an ISP.

The implications of this is that it makes much more sense to always keep airplane mode enabled and use public wifi only, if you can get away with it. You will not be geolocated by the cellular networks, though if you connect to public access points over wifi you can still be tracked by the network, other networks, and probably Google and Apple. It’s always best to connect via ethernet, but that is sometimes not possible.

Am I missing something here? Please keep in mind that I am not suggesting that you can communicate anonymously, rather, that you can hide your IMSI, IMEI, and location data. The communications themselves are unencrypted, and depending on what network the person you’re communicating with is on, the content and metadata of the calls/texts are known to the cellular network.

I discussed this in the GrapheneOS matrix server but I got a strawman response, it seems that I’m easily misunderstood. I’ve tried to be as clear as I can here.

EDIT: It turns out, I can send texts with airplane mode enabled. That means I can literally do everything with airplane mode enabled, that I can do without it enabled. As long as I’m using wifi, of course.

Again, to clarify - this means that I am not sharing my IMSI, IMEI or geolocation data, and if I’m using a VPN, it will become much harder to identify my identity.