Lines within a Business Group use a group-specific dialing plan, including short dialing codes for features and extension codes for telephones. Each line in a Business Group can be assigned an extension code, used within the group to identify that line.
Extension A is given a Extension code of 100 and Extension B is given a code of 101. In order for Extension A to call Extension B simply lift the hand set and dial 101. Wait for Extension B to pick up or go to Voice Mail.