Considering 2G is (globally?) deprecated I'm not surprised that it was open sourced. 3G/4G/5G are actively in use, so I'm guessing that's the reason they aren't available/open source (probably a lot of legislation to get that working).
16-Year-Old Builds His Own Open-Source Cell Phone Based On An ESP32
https://void.cat/d/5yraPL8tGHfurEjz5KUk7U.webp
The phone uses an ESP32 at its core, with a SIM800L GSM modem to interact with the cell network, including retrieving the system time. A small battery is included as well as all of the support circuitry for charging it as well as a USB interface that can communicate to a PC. The operating system for the phone is built from the ground up as well, with a touch screen interface allowing the user to make phone calls, send text messages, store contacts, and a few other basic features.
Yes, the modem is 2G, but I'd imagine it would not be too difficult to get 3G or 4G modems, but I'm suspecting that may not be open-source. Of course, this is a 100% self repairable phone.
It runs the paxos_8 operating system written in C, and it looks like he is actively updating it, as it received commits just yesterday.
See https://hackaday.com/2023/08/03/open-source-cell-phone-based-on-esp32/
#technology #opensource #ESP32
Discussion
I actually did find a 4G modem that looked like it was open source, so I think it is possible. He probably went with 2G because it was super cheap and still working in France.