FAQ/How to configure our pbx, so that the feature Record Missed Calls is active but not counting missed calls

From Snom User Wiki

Answer

The desired functionality is implemented the following way in firmware Version 7; note that the PBX has to send the "reason" field correctly according to RFC 3326 ("The Reason Header Field for the Session Initiation Protocol"):

Received ...:

CANCEL sip:...
CSeq: 8044 CANCEL
..
Reason: SIP;cause=200;text="Call completed elsewhere"
..

See also this FAQ article: When a call is picked up by a different extension my phone still displays a "Missed Call" message. How can this be changed?