Mystery socket
I seem to remember a generation of 56k modem cards having a phone-out as well as a phone-in socket. Idea being, I guess that you don't have two phone cords to your desk. No use for UK of course as we don't use RJ45 connectors on the wall outlets, but the old GPO-invented sockets, so a standard phone wouldn't plug in.
But that could well be what your mystery socket is...
Jim