How does an IP phone maintain its connection with the primary and secondary Cisco Unified
Communications Manager servers in the cluster?

A.
Cisco IP phones register with their primary server. When idle, the IP phones and Cisco Unified
Communications Manager exchange the signaling application keepalives. In addition, Cisco IP
phones establish a UDP session with their secondary server and exchange UDP keepalives.

B.
Cisco IP phones register with their primary server. When idle, the IP phones and Cisco Unified
Communications Manager exchange the signaling application keepalives. In addition, Cisco IP
phones establish a TCP session with their secondary server and exchange TCP keepalives.

C.
Cisco IP phones register with their primary and secondary servers. When idle, the IP phones
and Cisco Unified Communications Manager exchange the signaling application keepalives with
both servers using UDP keepalives.

D.
Cisco IP phones register with their primary and secondary servers. When idle, the IP phones
and Cisco Unified Communications Manager exchange the signaling application keepalives. In
addition, Cisco IP phones establish a TCP session with their secondary server and exchange TCP
keepalives.

Explanation:
Link : http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/port/8_5_1/portlist851.html
To connect with DB CUCM 8.5 use TCP
Cisco Unified Communications Manager System Guide Release 8.5(1), page-562.