Apple Devices, NTP-Requests?

Our DHCP delivers the address of an inteal (working) NTP-Server. However our firewall reports a "massive" Storm of NTP-Requests (from iPhones, iPads) trying to contact some server on the inteet. (it's blocked)

With 8 Devices on the network, it's like 1000 attempts per minute - why doesn't the iDevices take the inteal NTP delivered through the DHCP-Scope configuration?

Is this something we could change by configuration, or do the apple-things have hardcoded NTP-Servers?