Extension Mobility provides the ability to restrict access, here is a few details about the feature:-
The Extension Mobility feature enables a user to log in to an IP phone and automatically apply his or her profile to that phone, including extension number, speed dials, message waiting indicator (MWI) status, and calling privileges. This mechanism relies on the creation of a device profile associated with each Extension Mobility user. The device profile is effectively a virtual IP phone on which you can configure one or more lines and define calling privileges, speed dials, and so on.
When an IP phone is in the logged-out state, (that is, no Extension Mobility user has logged into it), the phone characteristics are determined by the device configuration page and the line configuration page(s). When a user logs in to an IP phone, the device configuration does not change, but the existing line configuration is saved in the Unified CM database and is replaced by the line configuration of the user's device profile.
One of the key benefits of Extension Mobility is that users can be reached at their own extensions regardless of where they are located, provided that they can log in to an IP phone controlled by the same Unified CM cluster. When Extension Mobility is applied to multisite deployments with centralized call processing, this capability is extended to multiple sites geographically separated from each other.
However, if you combine the Extension Mobility feature with the AAR feature described in the section on Automated Alternate Routing, some limitations exist. Consider the example shown in Figure 10-13, where Extension Mobility and AAR are deployed in a centralized call processing Unified CM cluster with one site in San Jose and one in New York.
Taken from the CUCM 6.x SRND:-
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/dialplan.html#wp1044461
Please also take a look at the CUCM 6.x features guide, it goes into more detail, including configuration:-
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/6_1_1/ccmfeat/fsem.html
Hope this helps
Allan.
