I don't recall the exact specifics, but there are limitations. One I think is the support for certain bands. There is an issue with VOLTE requirements, an example you can read about here, although I do not understand what it is. (Sorry for Gitlab lnk.) I've seen it show up on phones where the 4G and 5G indicators are located.

Not sure about graphene. I don't think anyone can use most of the Linux OSs here. Graphene seems to do well but you have to buy a Google phone. I use e/OS and as far as I know as of Dec 2025 only Murena and Mint Mobile will support it in the US.

Does the carrier somehow see your OS?

I'm not aware of any carrier that allows Postmarket. I have e/OS and there is only one service supporting that. I think Mint Mobile supports some of the alt OSs unofficially.

What's up with the carriers? Surely one of the big three have antennas on some channels that the phone have. Surely you can just pay 4x the cost to a reseller on that channel. (In the U.S. the big 3 have to sell their antenna time, it's where all the other carriers are).

Any carriers in US yet?