Let me say right away that this is my personal experience, in which I took some very radical decisions. If you are not ready for such decisions, just skip the relevant points. This post is specifically about smartphones; tablets (which do not have a SIM slot) can simply be reflashed with firmware such as GrapheneOS or LineageOS.
Disclaimer:
This is not a manual, but a story about my personal experience. I am not encouraging or urging anyone to repeat it. However, I will include the names of the tools and other necessary information (if you decide to repeat it in whole or in part) in this text. No links, this is not an advertisement, and I hope DDG hasn’t banned you 
Why?
I have always been concerned about the fact that virtually any phone can be tracked (for example, using triangulation) even after changing internet providers due to its static IMEI. As we know, most phones either do not support changing the IMEI at all, or it is extremely difficult and there is a chance of bricking the device. What prompted me to do this is that Google will soon make it more difficult to install apps from third-party sources, and some vendors (such as Samsung) have stopped supporting unlocked bootloaders, making it impossible to install “clean” firmware. Therefore, I consider any solutions that may currently exist for smartphones WITHOUT the ability to unlock the bootloader to be temporary. Plus, most smartphones have extremely limited update support in terms of time and speed, which forces you to buy newer and newer models. Well, it’s time to move on.
Step one. Organizing communication.
I decided to completely abandon phones as such. Even “dumb” ones. In the future, I will purchase a phone that runs entirely on Linux for the convenience of working with applications.
I decided to address the need to receive SMS messages for registration with government and other services and to be able to use LTE by purchasing a Gl.iNet Mudi v2 (E750v2). Why: it supports SMS reception with the ability to forward to email (natively) and change the IMEI via an AT command (AT+EGMR=1,7,“XXXXXXXXXXXXXX”) available directly in the web interface. Also, with a recent update, it automatically randomizes BSSID and MAC, making tracking more difficult. If you decide to do this, DO NOT change the IMEI too often, only when you change the SIM card. Changing the IMEI with the same SIM card is useless, as the new IMEI will have the old IMSI. For visualization, see the image below:
In my case, this is more than enough (I didn’t use regular calls before, and we require email and SMS for registration, so my method solves my problem).
Roughly speaking, this is enough to receive SMS messages to log into Signal or a bank (yes, hello to banks that still don’t support TOTP). For the rest, there is Signal/Telegram (I don’t recommend the latter, but since many people use it, alas).
And here we smoothly come to problem number 2. How do you register for Signal if you don’t have a mobile app? It does not allow (officially) registration exclusively from a PC.
Step two. Service applications that are not available on Linux or the Web.
For this, I needed the Waydroid tool. It is essentially a full-fledged virtual machine with Android that allows you to run Android apps directly on your computer. This may come in handy if, like me, you decide to buy a Linux-only phone, which some services consider a full-fledged desktop and do not allow registration, expecting a mobile OS.
Most applications worked fine, except for one regional one. But that was solved by “oh well, ditch it.” Fortunately, it wasn’t critical for me.
Step three. Backups.
Unlike mobile operating systems, Linux natively supports LUKS, so all the necessary flash drives with backup copies of documents and keys can be encrypted without workarounds in the form of third-party software. Full disk encryption.
I use LocalSend to transfer files between devices (it even supports creating a LAN web page for those who don’t have this application). I use an external drive to copy important information. No more dependence on cloud services, which arose due to the “disappearance” of SD slots in some phone models.
P.S:
I am also considering using SIP (VOIP) numbers for maximum privacy, but only if life without regular calls proves difficult. I also plan to buy a Pine64 phone, but for now, I will try to get by with just a computer, as Richard Matthew Stallman does.
I understand that there are a LOT of caveats with this configuration, but this is only the beginning of such a journey into the world of radical privacy. I think I’ll write an update if something happens that is impossible (or extremely difficult) to solve with my configuration.
Accordingly, these actions have allowed me to almost completely “move” from cloud services to a set of my own disks with data in different locations (redundancy). And at the moment, the only thing I use from cloud services is Proton (almost all of its tools).