What is Extend and Connect?
Cisco’s Extend and Connect feature was introduced into Communications Manager 9.1(1) and Jabber for Windows 9.1. It allows a user to define a telephone number as the desired destination. This number must be any destination number that can be dialled from CUCM.
A full description of the feature can be found here
The fact that the destination must be diallable from CUCM allows Extend and Connect to be used along by Jabber users who do not have a Cisco IP Phone. This then is a great way for enterprises to migrate from a legacy PABX to a full Cisco deployment while using Jabber as the preferred Instant Message client.