Please forgive me if I am wrong, but from what I know there is no way
to simulate a 'hangup' condition by asserting a sound. The telephone
spec calls for the tip-and-ring voltage to change. It is another thing
if some brain dead modem chip manufacturer looks at something other
then voltage swings. Do modem (or modem chip manufacturers) do that?
I would suggest the if you are interested in how the telephone tip and
ring scheme works you might want to look in back issues of Popular
Electronics (within the last two years). They had a telephone simulator,