To enable Netwrok Time Protocol (NTP) for certain phones, such as the Cisco Unified IP Phone 7911G, 7941G, 7941GE, 7961G, 7961GE, 7970G, and 7971GE, connected to Cisco Unified CME running SIP. By default the SIP phone sync with the CCME router.

voice register global
  ntp-server ip-address [mode {anycast | directedbroadcast | multicast | unicast}]