Every government office building I have ever worked in - and I have worked in plenty in 24 years - has had at least one electronics closet that contained the office's phone equipment and usually the computer servers/network equipment as well.
Sometimes that was in a converted closet if it was an older building, or a special room with a special kind of flooring if it was a newer building and sometime it was indeed in the basement. But, the phone equipment was never "perimeter of the office environment". If you're familiar with another setup, you must be referring to a branch of government that I'm not familiar.
Yep, there’s ALWAYS a phone closet.
Since 9/11/01 many changes have been made everywhere in government buildings.
Equipment of every kind will be within the lockable perimeter of the office complex. That is, you walk into the reception area, and even if the office area has a rear entrance, or wraps around a corner of the building, that reception area and the other exits/entrances are lockable, and most are usually locked except for exiting 24/7.
A server room will be part of a larger internal installation that is quite lockable, and probably is. The room with the PBX is required to be lockable, and it's required to be WITHIN the area of control of the responsible authority.
USPS provides more buildings than does GSA, and they operate with a set of standard guidelines that have been reviewed for all security needs by people with expertise in that area. GSA provides yet other buildings and they do the same. DOD has many facilities, and they provide additional security requirements which include "blast zones".
The various federal agencies that manage federal buildings of all kinds draw from the same agreed upon and reviewed security standards.
If you work at Justice Department, you're in one of a dozen buildings, and you'd be surprised how they actually control entrance, exit and protection of equipment of value.