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

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

Explanation:
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.