The data and messages and activites that your baseband processor in your mobile phone gets up to... nobody talks about. The manufacturers don't release documentation, the mobile phone networks don't either, and you cannot find out. They even don't disclose things in court, arguing against even high level discussion of what these things get up to.

These processors are not beholden to Android at all, they are totally independent, lower. They can listen in to your conversations, listen to your microphone (or use the speaker as a microphone) and Android would have no idea --- no App can protect you, no Android fork can protect you from these things. GrapheneOS does not protect you from this stuff.

And remember, mobile phone networks are mostly ancient unnecessary tech stacks. All we really need is Internet Protocol. We don't need SMS or calls, we can do VoIP and all kinds of messaging (like nostr or signal) over the Internet. All we really need these days is an Internet connection, not the old text/call stuff.

BUT if you remove your SIM and maybe even poke a hole through your baseband chip, you won't be able to get on the Internet while travelling, you will be stuck with WiFi.