Hello:
You can define partitions and DN for auto-registration phones. Navigate to the following location
In your call manager admin page
System--> Call Manager --> Select the server where you would like your "Auto-register" --> Under "Auto-Registration" fillout the DN's and partition you would like to.
Following is more reading
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/4_1_3/ccmsys/a02autor.html
HTH