There are two varieties of device files related to serial drivers, and for each port, there is one device file from each of them.
Minor numbers are identical for both types.
If you have your modem on one of the ports COM1 through COM4, its minor number will be the COM port number plus 63. If your setup is different from that, for example when using a board supporting multiple serial lines, please refer to the Serial Howto. Assume your modem is on COM2. Thus its minor number will be 65, and its major number will be 5 for dialing out. There should be a device cua1 which has these numbers.