For one Cisco ATA to access a specific FXS port on another Cisco ATA, you use an IP dialing procedure. The Cisco ATA originating the call must have the following information about the destination Cisco ATA:E.164 phone number of the destination FXS port
IP address of the destination Cisco ATA
To enable the IP dialing procedure, you need to set the IPDialPlan parameter to 1 to instruct the Cisco ATA to look for the destination IP address in the dialed digits