VoLTE and Wi-Fi Calling on BYOD accounts

So, I have a 1st gen Pixel. It clearly supports Wi-Fi Calling and VoLTE features.

Can someone please supply me with a very technical reason why this device isn't supported for these features, in the context of having a BYOD account? My understanding is that even if I had a Videotron supported device (like a Samsung Galaxy 8+) the fact I'm on BYOD means I don't qualify for these services, which strikes me as odd.