AASTRA 4420 IP Basic (SIP) for MX-ONE User Guide - Manual (Page 18)

Outgoing Calls
Dialog 4420 IP Basic 18
Monitoring
You can monitor the progress of an outgoing call.
With the handset on hook:
z
Dial the number.
Press
When the other party answers: u
Lift the handset.
Press to terminate a monitor call.
SOS calls (Emergency calls)
Note: The number for SOS calls might vary between
countries. z
Dial the SOS number.
Press
If this feature is enabled it is still possible to make an emergency
call even if the status lamp is lit. If a dialtone is heard it is possible to
make the call.