A lot of phone systems saturated themselves when things first shut down. Probably not physical copper constraints but the virtual interconnects between providers:
Through the 1990s (and prior) this was not uncommon, particularly on high-traffic days, often holidays (in the US: Mother's Day, Thanksgiving, and Christmas), when calling long-distance. Also during natural disasters, when local calls might also be affected.
There were also systems outages, such as the 1988 Hinsdale switching station which took out phone service to most of the Chicago area (both local and long-distance):
"1988 PHONE CRISIS TIED TO 1 BROKEN POWER LINE" (1988)
Turns out that that switch was a SPOF:
To make sure such a crisis doesn`t happen again, Illinois Bell Telephone Co. announced Friday it is embarking on a $80-million, five-year program to construct a complete duplicate telephone network system throughout its Chicago suburban operation and to redesign its fire protection systems.
A lot of phone systems saturated themselves when things first shut down. Probably not physical copper constraints but the virtual interconnects between providers:
https://productioncommunity.publicmobile.ca/t5/Get-Support/A...
https://archive.ph/D9qQ4