The busy lamp field (BLF) is a feature that shows the status of phones on your NEWT PBX. It's a visual indicator that shows which phones are in use, and can also show if a phone is idle or ringing. BLF is a useful tool for people who make a lot of phone calls, like receptionists, sales staff, and customer support representatives

For Poly or Grandstream desk phones, the extension line key on the phone will be green when the extension is available and red if the extension is currently in use.