Unable to call one particular number

This is more of a solution than a question. I've had three months of not being able to call one particular mobile number and have tried every solution known to man or beast including swapping alternately between 3G and 4G which appeared to help for a while. Finally, out of desperation, I even tried a change of SIM. I have just discovered that the simple act of getting the recipient to turn off Wifi calling has solved the problem so I'm posting this here in case it helps someone else. Using iPhone XS