A Line key is a direct connection to your DID/Extension (#) in which calls can be placed. When calls are put on hold they remain on your line.

Park Orbits allow the phone user to put a call onto the system for others to pick up. Unlike a hold this is not tied to the line the call originated on.

BLF's have multiple functions. The main function is to have a key that will directly connect to a users extension with a press of the button.
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When transferring a call, push transfer, the BLF of who you want to transfer to and transfer again.
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A secondary function is that you can visually know who is on a call when their button is lit a solid green color.

Speed Dial allows for an outside number and label to be created on the phone for quick calling.

A switch button allows for a function to be enabled or disabled.

A familiar example would be the Night Button, which allows a receptionist the ability to force the system into Night mode when early leave is desired.

In this case an Auto Attendant could be triggered or calls could be sent directly to Voice Mail.

Voicemail button for quick access to your messages.

Transfer button is used to quickly push calls to another user.

Redial allows you to review your History of Incoming and Outgoing calls.

Phonebook button for quick access to your contacts.

Hold places the caller in a state of waiting with music.

Headset turns on and off automatic syncing between your phone and your headset.

Mute your device, the other caller will still be heard.

Speaker phone option.

Volume up and down on current audio source.