Look no further than the APC. It is either defective or it is too small to sustain power to the system for a reasonable amount of time.
Emergency generators take some time to come up to full load. The better ones get the engine and generator full up before cutting the power through the transfer switch. In that time period which can be several seconds to several minutes, the UPS needs to be able to fully power the system until the generator takes on the load.
In the city, we have several clients that will call, from time to time, and alert us to a planned power outage. When that outage is for a prolonged period of time, beyond what the UPS can sustain, we dispatch a technician and power down the system. When the power is restored, we send the tech to power the system back up again. We have never had a client complain or not pay the bill. It's just good insurance.
The electrician has no obligation to go to every electrical device and power it down. That's the owner's responsibility. You should have been notified by the owner of the planned outage and then you could have dispatched and powered down the system. The electrician had to power down the entire building or, at least, the part that your telephone system was connected. The emergency generator transfer switch has to be between the commercial power and the load for the generator to work. That involves a planned power interruption.
Rcaman
