> You can't have that with phones.

Why not?

Obviously we don't have that. But what stops an open firmware (or even open hardware) GSM modem being built?

There are some open firmware, or partially open firmware projects, but they're more proof-of-concepts and not popular/widely-used. The problem is the FCC or corresponding local organization requires cell phones get regulatory approval, and open firmware (where just anybody could just download the source and modify a couple of numbers to violate regulations) doesn't jive with that.

https://hackaday.com/2022/07/12/open-firmware-for-pinephone-...