The following numbers should not be assigned within your system because they are reserved for system functions. 


411
Information
611
Customer Service
911
Emergency
933
911 Test Number
700-799
Used for Dynamic Call Parking Queues
7000-7999
These are unusable as 4-digit extension numbers because they conflict with the 7+extension (3 digit extension) rule to transfer a call directly to voicemail. If you do not use 3-digit extensions, then this range can be safely used for other purposes.
70000-79999
These are unusable as extension numbers because they conflict with the 7+extension (4 digit extensions) rule to transfer a call directly to voicemail.  If you do not use 4-digit extensions, then this range can be safely used for other purposes.
99XXX+
99 plus a users extension (3,4 or 5 digits) are reserved to work with Intercom features.
5000
Unregistered Login to VMail (prompted for acct no and password)
5001
Registered Login to VMail (prompted for password only)
5002
Hot Desking Sign In
5003
Requests the User's PIN then provides a 2nd dialtone (useful for calling from phones in public spaces)
8888
This User has a limited view in the PBX portal. When assigned the Office manager role, the 8888 User can only view the Home and Call History pages. Other Office manager functions are hidden.
XX
Please use 3, 4, and 5+ digit extensions. 2 digit extensions will cause setup issues.