Recently I got a call from a Condo Association I have worked for off and on over the years. They said the doorbuzzer/intercom system was repeatedly going out. This was a slightly more complicated than average system because it utilizes the phone lines. They had had it installed last summer. It worked fine all summer and fall but now was going out.
It turns out the new system has several transformers. The folks who installed it were not very good craftsmen, in my opinion. There was one outlet in the basement by the panels, and they plugged two cord strips into it and rigged the transformers with extension cords. That was pretty sloppy. But the kicker was, the outlet they plugged into was a GFCI outlet that had been installed to provide GFCI protection to the circuit that went outside and up the back porch to power the de-icing cable in the gutter. So when there was flooding and extreme moisture permeating those gutters, the GFCI protector tripped, as it should. You don't want 120v power "live" laying in soaking-wet metal gutters! Normally, after the severe moisture passed, one would simply re-set the GFCI protected outlet. It was never intended to be used for other loads, especially not an intercom system that controls the building entryway. Since there was an on/off switch right next to the GFCI outlet, so that the de-icing cables can be turned off during the summer months, I would have thought the idea of plugging an intercom system into it would be off the table. Just by the co-incidence that no one at that condo association was paying attention to these things, and the de-icing cable power was not turned off for the summer, that the system worked in the first place.
I installed an array of receptacles so that each transformer would be plugged in separately, and all of them are now powered by a new, dedicated circuit. Big improvement.
If you live in a condominium or multi-residence building, it is a good idea to be sure all of your critical systems, such as furnace or boiler, hot water heaters, and door buzzer/intercom systems are on separate dedicated circuits so that if something unrelated goes down, you are not left out in the cold!
TOP: How I found it. (The plug-strips had been plugged into the GFCI outlet next to the switch.
MIDDLE: Transformers and plug-strips temporarily hung during work.
BOTTOM: Finished Product.