Buttons:

L1-L6 : Phone lines

CFG : Configure : opens configuration window.

XFR : Transfer : use to transfer call.
Blind transfer : press XFR + enter new phone number + press XFR again
Non-blind transfer : press XFR + press line number

HLD : Hold : place call on hold

RE : Redial : dials last number dialed

AA : Auto Answer mode : automatically answers call

AC : Auto Conference mode : automatically places call in conference if used with AA

DND : Toggle Do-Not-Disturb mode

CNF : Conference mode : toggles call in or out of conference
All lines that are in conference mode will talk and listen to each other.
Conferences are created in the phone, independent of your PBX.
All 6 lines can be placed in conference mode.

MTE : Mute : toggle mute

REC : Record : records a call

VIDEO : Video : Enables video mode for current call

SPK : Speaker Mode : toggle speaker mode
Speaker phone mode is used when you're not using a headset.
The phone will operate in half-duplex mode.
When the inbound audio is over the defined threshold,
your mic is muted until the inbound audio is lower than the threshold
for the defined delay duration, to avoid feedback (echo).