Westermo industrial modems are recommended for use with Omron PLCs. This combination has been extensively used and tested. Set the Westermo DIP switches to 9600 baud, 7,E,2 protocol. Refer to Westermo installation manual for other DIP switch setting details.
Standard settings as below:
SW1 All off
SW2 All off
SW3 1 (2-wire) and 7 (DCD follows state of line carrier) On
SW4 2,3 (9600), 5 (7,E) and 8 (2 Stop bits) ON
SW5  4 (9600 line modulation) On

The modem to PLC connection must contain a cross-over lead as detailed below:

Modem 9 pin male  PLC 9 pin male

Pin 2   to Pin 3 
Pin 3   to Pin 2
Pin 5   to Pin 7
     Pin 7 short to Pin 9
Pin 6 short to Pin 7
Pins 1, 4 , 8 & 9 not used.  Pins 1, 4, 5, 6 and 8 not used.

Set the Change PLC, Sysmac Way, Network Type, Settings, Driver tab to 9600 baud, 7, E, 2 protocol.
Set the Modem tab to Standard Modem, 9600 baud, 7, E, 2 protocol. Select No Flow Control and set the "Connect To" options ie. Country code, Area code and Telephone Number. These "Connect To" settings are mandatory.

The Modem settings can be set to default in the OS "Control Panel", "Modem" section.

DIP switches should be set on the PLC where appropriate.
eg For CQM1H
Pin 5 On - Serial comms set to standard host link settings.
Pin 7 On - Device other than Procon connected to peripheral port.