Not disagreeing with you, but you’re papering over a lot of complexity.

Note that cellular radios are highly specialized and the filtering circuits are tuned to specific bands. It’s not exactly like having a software defined radio in your pocket.

Next, at the modem level, you’ll need to implement and then sideload custom firmware. Finally, you’ll need to expose the right APDUs to the kernel to manage the whole thing.

TBH it sounds like a fun side project, but my point is you need to repurpose a lot of different parts of the stack to accomplish what you want.

I was pushing on the walkie-talkie case to gain the maximum results from existing phones, that's a true emergency case.

You’re absolutely right that the 5G/LTE baseband is a black-box nightmare to repurpose. But I’m not looking to hack the cellular modem; I’m looking for the dormant '4x4 car' already available.

For instance, many chipsets have an integrated FM receiver that is essentially a high-sensitivity VHF radio. By taking the raw audio output and applying a Software Modem (AFSK/FSK) in the user-space, you bypass the kernel/firmware complexity entirely. You don’t need to sideload a modem driver if you treat the audio jack or the internal FM bus as your physical layer.

The 'complexity' is real if you try to fight the manufacturer's fences, but it vanishes if you understand the full stack. A pair of wired headphones becomes your dipole antenna, and the phone's CPU becomes your DSP engine. It’s not about rebuilding the Ferrari; it’s about realizing there’s a VHF engine hidden in the chassis that doesn't need 'permission' to receive bits. You just need a software demodulator the catch them, but for sending you'll need an external transmitter (an USB SDR or jack-to-FM).