WIFI calling has always been great, but since quite a while now it's been appalling. Looks like this is related to some update in the past. The call quality is really bad. Faint, inconsistent line quality and overall a pain to use due to the bad quality.
I'm on the latest (March 2022) update (there is no newer release available). I use it whilst I have my phone on flight mode. There are no other settings for WIFI calling other than off / on.
I've reset my network settings, reset the phone, all to no avail. My internet isn't the issue as it's more than fast enough to support WIFI calling. I also use Teams Calls with video and again, no issues whatsoever.
From the forum, this seems to be a Samsung issue and I'm hoping there is an actual fix out there. If not, then I'll be trying an Iphone to see how that works and if it does, I'll probably switch to Apple (albeit that I don't really want to do that).