The GRP2604 does not seem to persistently try to connect to the configured NTP server in case it is unaccessible. In my case the phone is booted at the same time as the internet router, and since the phone boots faster, it cannot immediately connect to the NTP server (for approx. 5 minutes). And it seems to stop trying, so that later on time is never synced. I mitigated it by setting a low NTP sync time interval, but this means unnecessarily frequent resyncing later on. Not such a big issue if own NTP server is used (for ex. built in UCM63xx), but I expect it to become an issue if the phone shall be configured as a remote device using the VPN-functionality to connect the the office (initiating the OpenVPN connection can become a problem if the phone’s system time is not correct).
Other phones (GRP2613, GXP1625) (all connected to the same UCM63xx as the GRP2604) additionally seem to sync time via the SIP signalling, but not the GRP2604.