Comment by blamazon

With modern 'system on a chip' modems, data goes from baseband to application silicon over shared memory via a proprietary interface:

https://en.wikipedia.org/wiki/Qualcomm_MSM_Interface

I think non SoC high performance modems use USB.

Regarding older or less integrated modems that do use the 232 interface, this may be illuminating:

https://fahrplan.events.ccc.de/congress/2011/Fahrplan/attach...