I've had both wifi calling and 4G calling working on my S7 for a few months but since downloading the latest android update last night both have now stopped working. Rebooted phone several times, turned wifi off and on, turned airplane mode on and off and checked still connected to EE APN but still not working.
Spoke to BT and they say they've had a few similar calls but nothing they can do and will have to wait for another update to come through to correct the problem but don't know how long I will have to wait!
Thanks for posting, unfortunately there is no way of saying if the next update will allow the services to work. Currently BT only support and will gaurantee that the S9/S9+ the A8(2018) and iPhones 5S and above with stock firmware to work.
I understand that those are the only handsets officially supported but the fact of the matter is that all was working fine until the latest update so there is obviously something in that update which has screwed my phone
As it is not BT's stock firmware that was tested to work you may find that the serivces will keep comming and going everytime Android issue an update.
I know thats not overly helpful but the last security update seem to add the serivce to some Android handsets and remove it from others.