When you are sure and confident that you have already configured DHCP server to support Lync phones however PIN authentication still not working with Polycom CX and VVX series, try to check the version of your NTP servers in the environment.
I had an issue before that AudioCodes 4xxHD phones are surprisingly working with PIN auth while Polycom CX and VVX weren’t.
With all the NTP servers listed, all of them replied version 3 only. And from the packet, it seems the clock is not synced.
Wireshark Trace from NTP version 4
By standard, NTP version 4 should be compatible with version 3, but for some reasons version 3 does not works well on Polycom phone. So ask your customer to setup a new NTPv4 to support your VVX or CX phones and add it to your DHCP configuration. PIN authentication should work after that.