Starting to implement D717 Deskphones running Firmware 10.1.141.10 (recommended 3CX version)

We notice that the phones don't use the NTP source (ip address) offered in the DHCP options (both 4 and 42)
What we see on network behaviour is that still a connection is being made to an IP from pool.ntp.org is (after loading phone application).

This behaviour is not as expected and not what the configuration manual says.

When I look in the phone Web User Interface, it mentions pool.ntp.org (greyed out)
When the phone boots, it shows the in DHCP options set correct NTP source, before loading the phone application

I have to mention that we offer 2 Ip addresses as time source.
(now testing with only 1 IP address)