Polycom SoundStation IP 5000 on Mitel MX-ONE 6.1

On Polycom:
To ensure there is no conflicting existing config:
Menu -> Settings -> (Password = 456) -> Admin Settings -> Reset to Defaults -> Reset Local Configuration
Menu -> Settings -> (Password = 456) -> Admin Settings -> Reset to Defaults -> Reset Device Settings

Whilst booting, select "Setup" key:
Password = 456
DHCP Menu -> Boot Server = Static
DHCP Menu -> Boot Server = FTP
DHCP Menu -> Server Address = 0.0.0.0
Save and reboot 

On Polycom:
Menu -> Settings -> (Password = 456) -> Admin Settings :-

CALL SERVER CONFIGURATION
Server 1:

• Address = MX-ONE Server IP addres
• Port = 5060 (or SIP signalling port)
• Register = Yes

Server 2: Unchanged

Outbound Proxy:

• Address = MX-ONE Server IP addres
• Port = 5060 (or SIP signalling port)

Save changes

LINE CONFIGURATION
Line:

• Display Name = Boardroom (or any other preferred arbitrary string)
• Address = 4583 [Auth user name (usually extension number)]
• Label = 4583 [Extension number]
• Line Type = Private
• Third Party Name =  4583 [Auth user name (usually extension number)]
• Line keys = unchanged
• Server1 = unchanged
• Server2 = unchanged
• Authorization:
• UserID =>  4583 [Auth user name (usually extension number)]
• Password = blank (unless set)

Save changes

Mitel MiVoice MX-ONE Programming:
extension -c -d 4583 --third-party-sip-client yes