I have a ThinkPad X200 that I bought new from Lenovo when you placed your order and they built it to your specs in early 2008. It was very expensive -- I even sprang for the cellular option (3G, before we knew about Gs). Also with the docking station, which I used every day, all day, as my office desktop. Undocked the computer and brought it home each night.
Replaced the 160gb hard drive with a faster 1tb one 12 years ago. Only had 4gb RAM because 4 more would have been something like $700. Ran Linux and TDE the whole time. And kept it in very good shape.
So tonight I sprang for two 4gb memory strips for an 8gb machine (now $35 rotal) and splurged $88 for a 1tb Samsung 870 Evo ssd. I did not buy it a cellular account, so it will finish its days with its cellular circuitry unused. I've never even seen a Linux cellular application.
You make me curious ....
Apparently *some* USB-connected Huawei 3G modems supported voice calls as USB audio device after you poke them with right AT command, see
but not all (modem hardware) can do this. And answer appeared only in .. 2018?
Link in answer lead to
So *may be* ModemManager is (or was ..) the software you looked for?