The Bulk-Modify Extension tool lets you search for and select a group of extensions, then modify specific settings at the same time.
Log into /admin -> Setup -> Manage -> Bulk-Modify Extensions.
To modify a set of extensions:
Add the extension(s) that you want to modify to the Phone Extensions box.
Under Phone-Extension Fields select the field(s) that you wish to modify -> Click Next.
Make the change -> Click Add
Review the changes to be made under Modifications to make sure it is correct. Once verified by you, click to Save Modifications.