AASTRA DT4x2 for MX-ONE User Guide EN - Manual (Page 124)

Reference
DT412/DT422/DT432 124
Phone signals
The phone is signalling certain events audibly and visually as
described below.
The phone also generates audible ring signals and alarm signals.
Signal Meaning
Red warning light Incoming call.
Message waiting.
Sound Meaning
Ring signal Incoming call.
Alarm signal 1 short beep: phone is out of range of base
system.
Message beep: Incoming message.
No-movement alarm or Man-down alarm
about to be sent (DT422 and DT432 only)
Key beep You hear this each time you press a key (when
selected).