Dave,
I was in LA Area in June and heard a MofW employee make a call on 6133 Mobiltel (output freq. 160.605) (the channel you hear the paging "chirps" on) (UP's description of what you call PBX) located on Sierra Peak during daylight hours, each day that I was there. I did not hear any other activity on any of the other channels (9412 Executive Mobiltel, 7056, & 6252). I contacted a UP Superintendent of Communications for the area and he told me that the UP Executive Mobiltel (9412) 160.290 has been taken off the air in most locations in the West due to expired base station licenses and since there is the MofW pair at usually the same site, with little use of the system these days and to cut costs, the executive sites are being retired for good. As far as the former SP sites such as Oat Mountain (above Chatsworth), they are no longer being maintained. If the license is still current, they are left on, but as they break down, they are no longer being repaired or replaced. If a UP Employee makes a repair ticket on a site not working right or working, Communications Dept. makes a decision as to either repair it or retire it, and the same holds true for an expiring base station license. He noted that the sites on Holiday Hill (above Wrightwood) & Kelso for both Mobiltel Sites (9412 & 6133) and the UP Dispatcher base (channel 27 or 160.515) have been completely retired because of overlapping sites elsewhere. This is to cut all costs of maintenance and operation. Hope this is helpful/useful.