How to configure our PBX so that the feature Record Missed Calls is active but not counting the current missed call?

Answer

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

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

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