Air conditioning killed the phones
Not so much an odd reason (failed air-con killed the servers), but the way circumstances lined up to kill the rest is funny,
The "server room" was a little cupboard in the corner, with a wall mounted AC and probably too much server hardware. The phone system's main box was in there too.
The AC had been slogging its guts out 24/7 for years, and eventually the heat exchanger was a solid block of ice and it gave up. This prompted the servers to overheat and die.
The heat from the servers eventually melted the ice in the AC, which dripped into the the phone system's box, which had been mounted directly under the AC, and killed that too.