I seem to recall data from california, where a good number of handsets get
knocked off-hook during earthquakes. AFAIK no switch ever crashed as a
result of the load.
But the phone system in Raleigh, North Carolina crashed for a day when
Garth Brooks tickets went on sale by phone. No matter where you called,
you got a busy signal IF you managed to get a dial-tone at all.
AlanC