Hi Jerry,
Cisco 8831 Conference phone is not supported on UC540 device. To configure the 8831 Phone we atleast need to have the IOS version 15.3(3)M version and for UC540 the last release is 15.1(4)M version.
Following is the document you can refer for this:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucme/admin/configuration/guide/cmeadm/cmebasic.html#36214
Kindly confirm if you've any further queries.
Regards
Akshay