I have this working my UCA Mobile. Once I pick Officelink, I get two choices under "Call Using" - Deskphone or Twin. You might have to have your Twinned number setup in the UCA and the Phone system itself under the Assoicated Destinations. We just installed the Mitel 5000 system and when trying to figure out the twinning, we discovered the phone system and UCA have to agree on the treatement of a call before it will do what you want. ie, twin.
Hope this helps.