Hi Bjorn,
One thing that you can try is changing the timer for simultaneous ringing of IP phone and mobile to 8-10 seconds so that it allows the call to be answered on the IP phone and only after the configured number of seconds it extends the call to remote destination. You can check the "Remote Destination Configuration" section of the following doc
http://www.cisco.com/c/en/us/support/docs/unified-communications/unified-communications-manager-callmanager/200447-Single-Number-Reach-Feature-for-Cisco-Un.html
Manish
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