Are you sure this is right? It's been a long time since I was
involved in telephony but I would think you go to off-hook state
for the duration of the call and that a "flash" would be a valid
*event* during an off-hook *state*. Flash during an on-hook state
would be invalid/ignored.
In the olden days you'd distinguish these with a copper slug on
the core of a relay. :-)
I guess just off-hook state would be ok since that's the default state
anyway during a call. In any event, a receiver will have to be prepared
to track hook state via either mechanism.