We had a WE 711B SxS PBX at a college radio station I worked at, with several 584 panels.
We had battery at both -48 and +24vdc, and all lamps, and 2 out of the 3 interrupters were 24vdc.
We could run the entire air chain, emergency lighting, and studio equipment off of an inverter from the -48 battery bank in the event of a power failure, in addition to the phone system.
We also used 24vdc for various amplifier modules, control switching, etc..
