I worked at a major medical center for 13 years, spanning the late70's to the early 90's. I was not in the telecom department, but I did have a very good idea of how the campus phone systems and paging systems operated. ;-)
When I first came aboard, the campus was served (??) by Centrex-CU off of an aging and quirky 101 ESS on campus. Many departments had their own (meaning serving them only, but owned and maintained by Ma Bell) key systems. There was a mix of rotary and touch-tone on campus.
Ma Bell actually stationed two techies semipermanently on campus. They had a small 'office' in a room off of a steam tunnel, with their stash of cables, KTU parts, etc.
Our main paging system was totally independent of Ma Bell. It was based on a huge honker Crown amplifier and hundreds of speakers around the campus. The operators had paging microphones on their consoles, but they did not connect with Ma Bell.
However, several departments had their own departmental paging systems which *WERE* connected to Ma Bell. In these cases a departmental amp would be fed from a 600 ohm feed from the main campus paging system and another twisted-pair 600 ohm feed from Ma Bell's key system. To page locally, a code was dialed on the departmental intercom key and that cut into the departmental paging system. Both rotary and touch-tone phones were supported. When TT was used for a page, it was often obvious with a very loud TT burst as the page began.
I don't remember any black box or dedicated interconnect device for these. IIRC, twisted pair frame wire ran right from the KTU punch block over to the amplifier cabinets.
They usually kept pages short and to the point. There were two special 'code' pages. 'Dr. Red' was a fire alarm, with the location Dr. Red was supposed to report to immediately being the location of the fire.
'Dr. Blue' was for code blue, or a cardiac incident which scrambled the code team.
We converted to Centrex-CO in the early 80's. Same phones, same dialing plan, but all extensions cabled to the CO about two miles away. (The campus straddled a CO boundary. Pay phones in some buildings were served out of a different CO than the regular office phones.)
In the late 80's the Med Center bought their own switch, a Definity of some sort. Almost everybody got their own 2500 style desk set with their own DID number. Just before I left, four-digit dialing gave way to five-digit dialing as the campus expanded. There were still many rotary dial phones on campus until the new installation.