Hi,
the user part is missing in an Invite message before the Refer which probably leads to this 400 message from UCM with following Cause Description: "Cause: Invalid information element contents (100)".
INVITE sip:192.168.217.236;user=phone SIP/2.0
To: <sip:192.168.217.236;user=phone>
It should be in the following syntax afaik:
To:<sip:userpart@destIP>;tag=to-tag
INVITE sip:userpart@destIP:destPort SIP/2.0
Did you also contact Yealink support?
BR,
Chris