I have the the same problem on o2 contract o2 blames Samsung last software update any ideas how to resolve as poor house network reception desperate to get this solved any help out there.
Go on to O2 chat to ensure it has been activated.
If it has take out your sim card and reboot your phone. Put your sim back in and reboot your phone.
You may have to reboot again.
Go to setings, Connections, Mobile networks, check if VoLTE calls is on.
If not turn it on, if you can't then something has gone wrong with you WIFI calls setup possibly from O2.
Try rebooting again
Open your Phone app
Go to settings and down at the bottom you should see WIFI calls. If not, a quick reboot
Turn on WIFI calls.
You should be good to go.
Lots of reboots requied I'm afraid.
Open the Phone app. Select settings and scroll down to WiFi Calling.
Turn on Airplane Mode. Try to make a call. A dialoge box should popup to allow you to enable the WiFi calliing feature.
Pull down the quick options from the top of the screen and swipe to the second screen. There may be the option to turn on WiFi calling. If not from the quick options type in the search bar WiFi calling and see if it brings up the option.