"Invalid certificate" errors on WiFi calling have gone from occasional to the norm. This happens both at work and at home, and I don't have anything at home that would alter traffic. I just tried it now with my router recording packets. 208.54.66.68 port 5061 produced certificate errors on four attempts. Eventually, a connection to 208.54.66.92 port 5061 worked. I'm guessing that you have some misconfigured servers or you haven't sent out all the valid certs to your phones.
I have a Sony XPERIA Z that is up to date.