This is unsupported (FAC) on Cisco Mobile 8 (Device Type: Dual Mode for iPhone) (Assumption: Call Manager ver 8.5)
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/8_5_1/ccmfeat/fsfaccmc.html#wp1071682
---------------
The minimum requirements for CMC and FAC specify that every server in the cluster must have Cisco Unified Communications Manager 5.0 or a later version.
The following Cisco Unified IP Phones (SCCP) support CMC and FAC:
•Cisco Unified IP Phones 6900
•Cisco Unified IP Phones 7900
-------------