A.7. Modem hangs up during initialization
Using setserial will reset the serial port. This will hang up the modem.
setserial is sometimes used during the boot process, resulting in the output seen in Figure A-3. Look into the file /etc/rc.serial and remove any references to the port which is being used as the serial console.
Figure A-3. setserial causes a modem to hang up as the machine initializes
Mounting local filesystems: [ OK ]
Turning on user and group quotas for local filesystems: [ OK ]
Enabling swap space: [ OK ]
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A