Make sure you are giving the modem option to pppd, so that a SIGHUP will
be delivered to pppd when the modem loses the carrier signal. Also, if
you are using an external modem, make sure the cable provides this
signal to your serial port.
Don't rule out the possibility that the modem is still asserting the CD
signal even though the phone line is idle. I guess if you can find the
CD ouput of your modem, you could measure the voltage on the line and
look at the CD signal behavior.
-- Joe Harvell jharvell@mail.utexas.edu. http://www.cs.utexas.edu./users/jharvell