Because the handset ended up in a “cradle”, there was almost always contact between the handset and the cradle before the switch cut off the phone. That was true even when someone was hanging up normally. There was a bit of a rattle as the phone went into the cradle. When someone slammed the phone down, that contact between the handset and cradle was much louder, but was cut off much more quickly. It wasn’t painfully loud, but the person on the other end was very aware that the phone had been slammed.