Way back when a wrote a couple chapters for Android Application Development I wrote about Android's RIL daemon and the underlying device-specific RIL libraries. It's gotten a lot more complicated, but the source code is open and includes what appears to be a reference implementation: https://github.com/android/platform_hardware_ril
I have not looked at this part of Android source code in depth in a while, but from a quick look it still looks very edifying about how this part of a smartphone works.
I have not looked at this part of Android source code in depth in a while, but from a quick look it still looks very edifying about how this part of a smartphone works.