So, can I connect a regular POTS phone via a USB thing? What are they called and
do they work with Linux? I assume that the skype phone just acts like a soundcard
... same with what I want?

Plug it in and see what differences it makes to arecord -l and aplay -l

POTS phone:
- expects a dc voltage across the line. Not so phantom, phantom power...
- expects 600ohm match... depends on how old it is though, newer may be less picky
- runs audio both ways on the same line... this may be more difficult to deal with. (echo cancelling)

Better to use two separate lines... prolly what the skype handset does.