Caller ID units connect via USB to the main terminal (or
whichever will be using the caller ID device).
They then connect directly to
your phone line and into the phone. There are two ports labelled phone and line. Disconnect your phone from the phone line socket and plug the caller here instead via the phone line cable picture here below, into the line socket.
Then plug the cable you just removed from the phone line socket into the phone socket of the caller ID.