I tried various settings and I have to conclude that there is something wrong in the latest Openwrt stable build with regards to Wifi calling.
I had to revert back to a non Openwrt firmware and it started working again.
I found two other people make such a notice
Wifi calling DNS request get rejected but why? - Installing and Using OpenWrt / Network and Wireless Configuration - OpenWrt Forum
Configuration guide for WiFi calling? - Installing and Using OpenWrt - OpenWrt Forum
I do not have any technical knowledge to help you troubleshoot but I hope someone can and maybe all these three complaints can be made into one for the future.
(Still working here on current release.)
(BTW, you provide no information - yes I understood you did not have the technical knowledge to "help"...but you say "latest stable". Can you at least mention the version?)
With little information (plus it's working for others), that's gonna be difficult.
Also, it's clear both threads are unrelated issues - they merely resulted in the same problem for the user. In fact, in one thread OpenWrt works and other devices don't. So your links confused me.
FWIW, I've never noticed any problems with wifi calling and OpenWrt.
I've used OpenWrt for years, including LEDE. I'm currently on OpenWrt 21.02.3 (Current Stable).
Wifi Calling continues to always work fine for me, with zero configuration needed (it is an outgoing IPsec tunnel anyway, so shouldnt need any). Depending on how I'm connecting to my ISP, whether its connected to an ISP router or bridged modem, or sometimes I'm doing double NAT - wifi calling still always just works in any situation on Openwrt.
(I'm in the UK with Plusnet ADSL as my ISP, and have a contract Vodafone Android phone, and a contract o2 iPhone, and they both use wifi calling all the time at home.)