It's Samsung software update causing the problem on mine. Samsung have cofirmed this but not told me when they will fix it. For now it will work if you turn off 4g. A pain.
On my phone, I found the setting under Notification Settings, drag the settings bar down to showing six settings, drag it down againg to expose 12 settings with three dots for three pages, move the settings left to the third page and sitting all by itself is the wifi calling button to turn on or off. It's fairly obvious that it is very lonely as nobody can find it.
The wifi drop down dos not inclue wifi callling. Its a feature of the phone first; then the service provider turns it on.
So may online reply and have no idea how this works. Why my new s9+ has this missing.??
I found the issue was inserting the sim card when the phone was turned on. I turned the phone off, took the sim card in and out again, rebooted and wifi calling appeared - in various places, including swiping from the top. I assume various functions are enabled when network checks are done at start up.
I knew i had wifi calling enabled with O2 because i had it on my previous iPhone. Just upgraded to the S10+ and live in an area with horrendous mobile reception.
Same problem. It thought it was an update that knocked it out...
but I figured it out - I roamed in another country... that its provider didn't allow. Once I cam back it still didn't allow.
I turned off phone.. took out my sim card... restarted phone.. re installed sim card - now it works.