[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...

[mmac_]

yeah this looks correct and it's a different world if you have direct access to the modem IC.

To clarify I was referring to the modules that the average person can get their hands on. If you rip open the module (or look at the FCC paperwork) you'll likely see a Qualcomm chipset underneath. It's just a level of abstraction really.