Long delay in getting here, but things are sorted now. The solutions was unbelievable simple: I went and bought a 2Degree simcard, put that into the phone (not other changes) and WiFiCalling is working absolutely fine. The phone is still not on any of the providers 'tested/approved' list, but according to 2D they have WiFiCalling enabled and as long as the hardware makes he right noises it will be allowed to work.
Thanks to 2D - I am now a delighted new customer ;-)