This field stores an alternate X.25 address that your device accepts on incoming calls when matching traffic to this IP address, providing flexibility if the remote system calls from a different number. Network engineers use this for redundancy or when the far-end X.25 switch may route calls from multiple X.25 numbers to the same destination. If the primary X.25 address is '311061234567' and alternate is '311061234568,' your device accepts calls from either and maps both to the same IP address.