Hi, I have exactly the same problem using an iPhone X. Poor network coverage at home even though the network map says its good. WiFi calls repeatedly drop either within the first minute of the call or after a few minutes. I have good fast fibre broadband which is reliable. After call drops I can't resume WiFi call as WiFi calling has gone from home screen. I resort to making conventional call over network from upstairs room standing close to a window!!
Have contacted o2 a number of times over this issue and they don't seem to be able to help.
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