Telegram literally stores all your messages, metadata, etc. in plain text on their servers. This means that it provides considerably worse security than even proprietary messengers, such as WhatsApp and Facebook Messenger. Telegram has an option for encrypted chats, but it’s not available for groups, lacks support for voice and video calls, and Telegram deliberately goes out of their way to make the experience of using encrypted chats as painful as possible.

You’re even better off using WhatsApp, but if you actually want a good messenger, switch to Signal. It’s free and open source (both the clients and the backend server), developed by a nonprofit organization, and it’s basically the gold standard for encrypted communications.

All of these are insecure as hell. Linux phones especially https://madaidans-insecurities.github.io/linux-phones.html

Fairphone also really fucked up: They signed their own OS with the publicly available (!) AOSP test signing keys. These guys really don’t know that they’re doing, and I would trust their hardware or software whatsoever. And no, installing a custom ROM doesn’t solve this. Considering how bad their security practices are, we genuinely have to assume that there are security issues with the device firmware as well.

/e/OS is based on the already insecure LineageOS, and it weakens the security further, so it’s not a good option either.

None of the options you mentioned can be compared to GrapheneOS. It’s currently the best option if you value your privacy and security. You don’t have to give Google money either, since you can just buy a used device, which is also cheaper and more environmentally friendly. Google also makes repairing their devices pretty easy for consumers and even works with iFixit. Here’s a Mastodon post I recently saw about that: https://social.linux.pizza/@midtsveen/113630773097519792

I don’t think it’s a coincidence that the shittiest companies are those, who enforce Google’s broken and monopolistic “Play Integrity” API. Revolut has connections to Russia, McDonalds supports the Israeli genocide in Palestine and Authy has always just been a massive piece of shit, not even allowing users to export their TOTP seeds. These are three companies I would NEVER even consider using anyway.

And “Play Integrity” API actually does NOTHING, absolutely NOTHING for your security as an end user.
You use an outdated, unpatched Android version with multiple severe, publicly known exploits on an insecure device?
Google doesn’t give a single fuck.
You use the newest version of Android with all the patches applied on Google’s own hardware, with a locked boot loader and a hardened operating system?
That’s not allowed by the “Play Integrity” API.
It’s only purpose is to serve Google’s monopolistic business interests.