When the IP Office is restarted, each extension returns to the settings of its default associated user, unless that user is set to Force Login (User | Telephone)
If another user logs onto an extension, when they log off, the extension returns to its default associated user unless they have logged on elsewhere or are set to Force Login.
In the US the Base Extension number is used for E911 calls. Any change to an extension's base extension must be matched by changes to the E911 adjunct database, see E911 Overview. Extensions without a Base Extension cannot be used with E911 and are associated with the NoUser user.