Which method does a Cisco Unified 9971 phone use to send keep-alive messages to Cisco Unified Communications Manager? 

Answer options:

A. SIP NOTIFY with Event set to keep-alive
B. SIP OPTIONS
C. SIP REGISTER with Expires set to zero
D. SCCP StationRegister
E. SCCP StationServerReq

C

Phone registers with primary and establishes keepalive connection with secondary. Expires = 0 keepalive mechanism allows Cisco SIP Phones to more closely resemble the failover / fallback behavior of SCCP.