[mmjaa]

The 'reason' for this distinction between breaking out the audio of the modem to a standard interface, versus not breaking it out - but rather requiring a secondary processor to submit proprietary packets - is simply this: cell equipment manufacturers have to comply with the requirement that it be relatively difficult to record calls made with their equipment.

Putting the DAC elsewhere absolves them of this requirement and drastically simplifies their time-to-market statistics. If you make the OS and other layers of the stack responsible for the ADAC, then you don't need to worry about it - as long as there is a usable interface.

Around the world, different markets have regulations governing the recording of calls. Its never supposed to be easy to do this, at a hardware level, because its supposed to be very hard even for the players in various markets to do this - without a third party (i.e. government) involved.

tl;dr the reason this is so, is because hardware vendors have to comply with legislation, and doing squirrely ADAC instead of out of the box Audio routing absolves them of the responsibility.