Admin left a document on IP-ATS. Excerpts from:

The number (extensions)
100 : Number registered for the phone that is connected to the gateway to port 0/0
101 : Number registered for the phone that is connected to the gateway to port 0/1
102 : Number registered for the phone that is connected to the gateway to port 1/2
103 : Number registered for a phone that is connected to the gateway to port 0/3
0666: A number registered for a ritual. can be removed. His own hand did not rise.