To access the dialog box for this setting, have no RTU selected and select Options from the top menu. From there select Modem:
In order to make a remote connection between Alarmware™ and an RTU via your computer modem, you will need to properly set the COM Port Number that your computer's modem uses. If you do not know which communication port your modem uses, go to the Windows Control Panel and select Modems. Click on the Diagnostics tab of the Modems Properties dialog box. Note to which port your modem is installed. Set your Port Number to match this setting.