Make a companion shared line appearance of the DN in question on your own instrument.... No, not really scalable, but should work on a onsie-twosie basis. For monitoring the on/off hook status of multiple sets you could expand this idea by giving yourself a 7965 (or 7975) then adding one or two expansion modules (sidecars). This gets a little expensive however.
Beyond that I can personally recommend 3rd party software by the name of "Phone Remote" from Voip Integration (dot com). We tried it and liked it so well that we bought a cluster license for it. ($499 individual licenses, $2000 for a cluster lic)
Original MUG/NAMU Charter Member