Hi,
So if your endpoint is provisioned with CUCM (that means config will be pushed by CUCM) then it will take the time as configured in CUCM Date time group and in that case it will override the NTP config on endpoint and make the NTP mode as Auto.
But if you have just done the SIP configuration and set the CUCM as proxy on endpoint in that case the endpoint NTP config will take preference and it will not use CUCM NTP.
Regarding your endpoint 20 mins ahead, please check whether it is properly provisioned with CUCM or not.
Please let me know if it is helpful.
Thanks