What I used on CUCM6
Setup NTP on in the Cisco Call Manager (CUCM), so the CUCM and phones always have the correct time!
The phones are connected / linked to a device pool in de Callmanager
CUCM > System > Date/Time group [ CMlocal ]
The time/date format on the phone is set in this field
CUCM > System > Device Pool (one time setup)
Setup the time of the CUCM (+ phones)
CUCM -> "Navigation" => Cisco Unified OS Administration
Settings > fill in "Time" + "NTP Servers"
Time This is the local time CUCM + phone will use
BUT use NTP instead!
NTP Server : fill in one, or better 2 ip addresses of local NTP servers
Try your internal Microsoft domain controllers or internet NTP servers. for example the ntp server of you provider and an extra backup (but it is possile you have to change firewall settings)
Note, what I found in the documentation
CUCM only works with a Stratum > 10 and NTP Version 4
Guido