Wifi Calls drop after a few minutes in my home. It happens on my wife's Galaxy S8+ and it happens on my new Note 9. I'm really thinking it's something with the router in my home. Just in case it's the same as yours I have a Linksys EA7300. Firmware is up to date also.
I have very bad cell reception in my home so it always connects to calls over wifi and I can see that due to the wifi icon on the calls and call history screen. It all sounds great until it just suddenly drops the call a few minutes in and then if I try to call the person back right away it won't even try to ring. I have to then turn off my phone's wifi in order to get a call through. After some time re-enabling the wifi lets it work some of the time again.
In addition, I do have fast 100MB/s + speeds and it's a very strong wifi signal in my home.
I'm thinking it's a port forwarding or blocking issue perhaps.