Why no VoLTE when switching from Contract to BYOD?

I'm sure it's a general statement to say that unlocked android devices might not be compatible with their VoLTE... so they won't activate this feature on BYOD plans since you are more likely to have a device not from Videotron on those plans. 

I'm sure you can call and ask Videotron to enable VoLTE. 

Or one thing you can try is first make sure you have a VoLTE sim (the one that came in the yellow card)... after that, call videotron and tell them that you have an iPhone 7 and would like to activate the VoLTE service. 

This is what I did when my wife had an iPhone 6s Plus (which is not a supported VoLTE device with videotron)... and when I switched her to an iPhone 7 Plus one month later, VoLTE was there "automatically". 

that said, how did you switch to a BYOD plan? usually, they don't allow that kind of switch unless maybe if you open a new line and transfer your "old" line/number to this new line.