AASTRA 4420 IP Basic (SIP) for MX-ONE User Guide - Manual (Page 41)

41 Dialog 4420 IP Basic
Messages
Messages
Manual Message Waiting (MMW)
If the called extension does not answer you can initiate a message
waiting indication on that extension (if this feature is allowed). If
there is a message waiting, the Message key lamp is on and you
will hear a special dial tone after lifting the handset.
Answer
Message
Х Ф
Press.
A call is initiated to the extension that requested Message Waiting.
Note: After the call you have to cancel the message
manually, see below.
Cancel MMW at your own extension #31#
Dial.
Press.