Some further research reveals that it is often not as simple as changing network, what appears to be happening is that many of the networks say they support wifi calling on a certain model but if you dig down that often only applies if you buy the phone directly from them with thier own permissions set on the phone. Unless you have an Apple phone in which case apple won't let net sell them unless the wifi calling feature is enabled.
Its a real shame that networks are using this as a ploy to limit functionality of a product on their network unless you buy it from them, for example I bought my g60s directly from motorola not ebay or some shabby third party retailer, but again the wifi calling is not enabled.
I am wondering if this a money supermarket or which issue, its just more people should be aware that you can buy a phone from a manufacturer but some networks disable some of its features unless you buy it directly from them, im not saying Virgin do this deliberately but this also seems to be the case with a lot of networks
Usually it's not that networks are actively blocking third party devices from working with WiFi Calling or disabling features, it's that for WiFi Calling to work on a device, then that WiFi Calling settings for that mobile network need to be in the device. WiFi Calling for Virgin Mobile has different settings than O2, EE, Vodafone or Three.
For Android, Mobile networks can only add these settings with manufacturers they have relationship with, which usually means the devices they directly sell. As Virgin Mobile don't sell Motorola devices, then WiFi Calling setting for Virgin Mobile aren't in the Motorola firmware. Even where it is added it will usually be for launch devices and later, and won't usually be backwards compatible into earlier devices.
Apple devices work differently to Android in that the Carrier Settings option that makes WiFi Calling work is available on all devices running the minimum iOS version for the WiFi Calling option so gets added to all iPhones that run that supported OS version.
I work for Virgin Media - but all opinions posted here are my own