I'm surprised they were a little brusque with you given how polite you were with them
. I work in a much higher level of support and am paid far more than those guys but have to admit that despite the level of professionalism I am required to adhere to people who are polite tend to receive a higher level of service on account of the fact I'm a human being, not a robot.
WiFi calling is not supported on Virgin Mobile. They do have the 'Smart Call' app that allows use of a Virgin Media home phone package via an app but not WiFi calling per se.
They were actually quite accurate when they said that if the device supports it you can use it. The Lumia 640 they sell does not support it as it requires specific firmware which is not present on their version of the device. They do not advertise this feature on the website.
This is the reason why Three also do not support it - it's not enabled in the firmware. The network operator pushes an OTA firmware update to enable it on devices and provisions them.
I imagine ZR, being a human being
like everyone else manning support services, is unlikely to work 8am - 10pm without breaks. So they may have simply been done for the day or been taking lunch.