Last we heard Zong's VoLTE was still internal and only for employees and you can only get it enabled on your number if you know someone working for Zong.Zong, you say? They've launched VoLTE? I don't think anyone here mentioned it (with proof).
Also, how many of these operators support VoLTE on the iPhone, btw?
As for the Pixel 4 - it's a shame it's SD 855... which makes me raise this question: is this chipset-dependent, or IMEI-dependent?