There are very few modems that support exposing the audio data in any other format than an electrical PCM interface, see my blog post form 2017 at https://laforge.gnumonks.org/blog/20170902-cellular_modems-voice/
The situation unfortunately didn't change meanwhile, so we still are stuck with that.
The only vendor that persistently supports PCM audio over USB (at least according to technical documentation) is SIMCOM. I've been trying to get it to work, but unfortuantely failed so far.
I can establish the voice call and issue the AT+CPCMREG=1. From that moment on, there also is data coming out of the /dev/ttyUSB device, but the data rate is much too low, and it's very bursty. Interestingly, the amount of data readable from that /dev/ttyUSB device depends on whether or not there is voice activity in the channel, i.e. during quiet phases, there is very little data, and when there is voice data from the remote end, there is more data to read. This is not at all what one would expect from a S16LE 8kHz PCM sample stream.
When looping the data (e.g. with dd if=/dev/ttyUSB9 of=/dev/ttyUSB9), all I get in the audio channel is some strange noise.
Likewise, http://settrans.net/~rah/misc/simcom-tone.c doesn't play any tone to the remote side, nor does it record any audio.
I currently only have a SIM7230E available, which is already discontinued. The fact that SIMcom doesn't seem to be making any firmware updates available doesn't help the situation, either.
I guess I'll wait for some other SIMcom modem models to arrive, which I ordered two days ago.
