I'm answering my own question insofar as I was given the opportunity to be able to do the usage checks myself, so yes this HTC 10 supports VoLTE and WiFi Calling "out of the box". So no need to buy your device via Swisscom or another operator, no need for special firmware from the operator, the unlocked version has everything. Thank you Digitec for allowing me to check the phone thoroughly on site and thank you for the advice and the great welcome.