It was called a Candella Circuit and it was very unique when it came to monitoring fire alarms basically it was a board with lights that lighted up when a contact on Fire Panel closed and it was used well into late 1980's by the City of Pittsburgh as well as other city's and towns . There was no supervision and you had no idea if wiring was even working until you did monthly building test.
But it only lasted a few years again due to the high cost of maintaining it and poor quality of phone lines in many areas at the time and was turned over to Guardian Protection who eventually abandoned it.
Finally Fire Alarms where allowed to report in via Digital Dialer which used POT or Plain old Telephone Lines which at first there where only 3UL Listed central stations permitted to send signals to city which was Honeywell, ADT and Guardian Protection . But then the city of Pittsburgh finally allowed all centrals to receive and send signals to them in early 90's. All in All it was a learning curve for Fire Officials as technology and codes are constantly changing and facing the massive amount of Fire and Burglar Alarms the city has they did a good job administering them. Bottom line they kept Pittsburgh Citizens safe
Amazingly even thou the Candella system was unsupervised there where No incidents where it did not work that I know of because Bell Telephone Employees did an excellent Job in maintaining and checking the lines . Which where marked with special red caps anywhere they terminated.Unlike today where Verizon is trying to get out of hardwired or POTS phone lines
Of course today there are many ways to send in Fire Alarm signals from Digital Dialer's over POTS to Cellular and Internet Communicators and even Private 2 way radio