I have the same issue with both iphone 8 plus on my account. I spoke with technical support a few times and once with apple. In the end, technical support advised me that they are aware there is an issue affecting people who purchased their iphone outside of videotron and would call me back in a few days (hopefully with a solution)
Did this get resolved? I have the exact same issue with my iPhone X. Really frustrating!
do you have VoLTE enabled and working on your iPhone X? this is the first thing to get... without this, Wifi calling will not work.
You first need a compatible sim (the one that came in an yellow card and shoes NFC on this card). and then, you need VoLTE added to your account.
I have a friend with an iPhone 8 not bought with Videotron... he got VoLTE and Wifi calling enabled and working.
Same for my wifi's iPhone 7 Plus... bought somewhere else... and VoLTE and wifi calling working after getting the new sim and VOLTE added to her line.