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