Unable to assign a particular extension number to a user from Admin Console. An error is shown, "User settings saved with errors. Failed at extension update." or an error indicating that the extension already exists. Other extension numbers (such as the suggested extension) can be assigned to the user.

Use a different extension number.

Some extension numbers are reserved for use by the system for use by services. These are not always visible in Admin Console. For example, 601-605 are reserved for Call Park extensions.