Sounds like there's still quite a few 5ESS systems around. Yes, they're now supported by Nokia. We've got many ISDN and or Avaya 8510 phones on the system. Unlike the 8510s on the Avaya PBXs I've worked with, these are powered by a power supply typically in the IDF closet. I was surprised that the branches in other parts of the state don't have a 5ESS on site in their buildings, but are served by the local CO, including 8510 phones. Those use a 'U' interface card, meaning 2 wire. At the site there's an NT1 interface that converts that to 4 wire to the phone (+ the power pair). Some of the more remote buildings at the main site use the U interface, while closer runs use 4 wire, or a 'T' interface.

Interesting copper retirement projects are going on that will make a CO like this pretty much obsolete. In the mean time, good to know there's still a place for tip & ring and older telecom guys like me. Of course I work with VoIP, but the old switches will be around for a while. The voicemail system is Cisco, Octel up until several years ago.