Hi TB,
If you want to ust Mobile access on a Sip gateway take a look at the release notes for CM 7.1.3. SIP and H323 are now support for Mobile access.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/rel_notes/7_1_3/cucm-rel_notes-713.html#wp1602136
Steve Slater