Been ages since I've worked with the DK's, so this is a bit of a shot in the dark with a blindfold on.

Do you happen to have an expansion box with a PIOU (?) in it - that could be used for paging. I forget the exact card number, but our DK40 used one version for remote programming, but I remember it had a single paging output which would have had a twisted pair connected to screw terminals. Our multicabinet DK280 had a different version which had multiple paging zones and an amphenol connector.
Both cards would have required a transformer to convert from 600 ohm twisted pair and connect to your RCA plug.

Question for the experts: what programs are used to disable the internal page port and use the PIOU?