What I want to do is yank the line out of the telephone on my desk and shove it in to the FXO port of the CME router.
Now I've got the picture...
From everything I've seen (which is admittedly little -- long ago EoL'd technology), it's a digital PBX system. As such, if you plug it into a Cisco FXO port, there's a very good chance you'll fry the port. (I'm not going to tell you how many ports I've seen damaged by people digital phone systems into them.)
My advice... don't try it on anything you aren't willing to replace. Try it on a buttset or old analog phone first.
PS: a POTS line is a single pair. tip and ring. I know several PBX systems send power down an extra pair. The FXO card shouldn't have any connections on those pairs, but might!