After some discussion the service provider has admitted that the SIM they issue does not support WiFi calling. Apparently they are in the process of selling out to EE but that won't in itself allow WiFi calling to function. If we want to enable WiFi calling my wife can swap to a proper EE SIM for a small increase in monthly contract charge. The other option is a further small increase in cost and to swap to my service provider which will reduce the tariff on her energy bills. Before doing either of these I will put my SIM in the J3 and see what happens.
Thanks.
Rans6..................