Nortel NT8B91 Busy Lamp Field for M7310 (Grey/Refurbished)This Norstar BLF goes onto an Norstar M7310 and displays for the user whether the other phones on the system are in use. This helps the user deal with calls more efficiently. Add a Nortsar Busy Lamp Field (BLF) to the M7310 and see the status of as many as 24 telephones. This Norstar Busy Lamp Field (BLF) is excellent for receptionist and administrative assistant positions that need to know whether a phone is idle, busy, or on "Do Not Disturb" before