A.4. Modem hangs up upon reboot
During reboot the serial controller is reset. This drops the
modem control line Data Terminal Ready. This in turn instructs the
modem to hang up.
Avoid the temptation to configure the modem to ignore
DTR. This leads to a worse bug, where the
telephone line does not clear down correctly, the modem is engaged,
and there is no way to clear it. Ignoring DTR
also gives no way to clear hostile callers from the line.
You may wish to record the amount of time that the computer
takes from Restarting system to
the boot loader prompt.
The modem may also hang up during the boot process (as the
serial chip is reset) or when the init
run level is changed (as getty is
restarted).
Take our 
