Unable to assign a particular extension number to a user from Configuration Manager
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
Virtual Office Configuration Manager
Use an alternative extension number
Some extension numbers are reserved for use by the system for use by services, these are not always visible in Configuration Manager.
For example 601-605 are reserved for call park virtual extensions.