Exam 300-815 topic 1 question 46 discussion

The rerouting of calls requires using a destination number that can be routed through the alternate network (for example, the PSTN). Automated Alternate Routing (AAR) uses the dialed digits to establish the on-cluster destination of the call and then combines them with the called party's AAR Destination Mask; if it is not configured, the External Phone Number Mask is used instead. The combination of the dialed digits and the applicable mask must yield a fully qualified number that can be routed by the alternate network. https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab10/collab10/dialplan.html

The two masks that AAR can apply when routing the call through the PSTN are: The External Phone Number Mask The AAR Destination Mask The External Phone Number Mask is a mask that is used to manipulate the called party number. This mask can be used to add or remove digits from the called party number. The AAR Destination Mask is a mask that is used to manipulate the destination number. This mask can be used to add or remove digits from the destination number. AAR can also apply the dial prefix defined under the AAA Group when routing the call through the PSTN.