Is It Possible To Connect An External PLC To CX-Supervisor Via A Modem?

Yes, you can connect from CX-Supervisor via CX-Server to a PLC via a modem. In the PLC configuration 'Network Settings' dialog, select the 'Modem' tab. CX-Server uses standard TAPI build into the OS. The modem must be installed properly to be shown in the Modem list.