Time is not sync on my Cisco 7960


I use FreePBX with Asterisk (Ver. 13.7.0) and OSS Endpoint Template Manager.
Our phones are Cisco 7960 with POS3-8-12-00 firmware.

Since we migrated from a Trixbox Voip server to FreePBX, the time on the phone seem not synched.
When I restart the phone, the time is OK but after somes weeks, there is a difference of 2-3 minutes.
The time is sync on the FreePBX server perfectly.

I entered the right ntp server in Time Server (NTP Server) on OSS Endpoint Template Manager.

What should I check ?

Thanks for your answer and your help.