I was playing around with Yealink Phones, when I noticed, that in factory defaults, those Phones have hardcoded the CN-Pool as NTP Server.
Firmware T48-126.96.36.199, not the newest, but maybe 1 or 2 Versions out.
I have tested this on T48G, T46G and T41P, but it is most likely also the case for the entire SIP-T-series(T5x, T4x, T2x).
I couldn´t find any engineering contact at Yealink(I only have one sales contact), so if anyone is working with yealink might have better luck.
There are of course many providers out there using yealink phones and customizing them(like Deutsche Telekom, BT, nfon,…) and might configure different ntp servers, but many may not.