Sometimes the symbolic link /dev/modem is used instead of the actual modem device.

To specify a modem port choose:
/dev/ttyS0 for COM1
/dev/ttyS1 for COM2
/dev/ttyS2 for COM3
/dev/ttyS3 for COM4

Also in the Device panel:

Flow Control
Choose from Hardware (CRTSCTS), Software (XON/XOFF) or no flow control. The recommended setting is Hardware flow control.

Line Termination
Most modems use CR/LF but some models require the other options.