I have some of these sprinkled around the house in rectangular housings on the baseboards. The house had working telephones when we bought it, but we have been making do with cell phones. and the landline phone is not working at this point. (We never connected with the local telco.) The house had more than one line at one time. There are also standard modular phone plugs in several places.
I am fixing floors and I have to decide if they are necessary or desirable. If it is all junk, I should cut them off and not take the trouble to retain them with the new baseboards and floors. (They come up through the floors.)
Thanks for looking!