This applies to Cisco Call Manager deployments that are not using LDAP integration for their corporate directory. If you are using LDAP this Cisco document may help.
Scenario – The corporate directory as seen on your phones is setup to display the last 4 digits of the users number. Some numbers appear with 10 digits causing problems when dialing from within the directory.
Login to your CM Administration page and click on User Management – End User
Search for the User in question and click on the User ID Hyperlink.
The 9th field down under “Telephone Number” should be the 4 digit extension you would like to appear in the directory.
Here’s a little quote from Cisco……..
In Cisco Unified Communications Manager Administration, you can access directory information about end users from the End User Configuration window (User Management > End User). If you do not enable LDAP synchronization, you use this window to add, update, and delete user information such as user ID, password, and device association. If you enable LDAP synchronization, you cannot add an end user, delete an end user, or change some existing user information, including user IDs, in the End User Configuration windows.
As always I hope this helps. and feel free to leave a comment and let me know.