[Google Pixel 3 XL Questions & Answers] deusfaux:
SOLVED.

~30 mins on the phone with Telus tech support and explaining how provisioning the billing codes on my account for VoLTE/WiFi calling wasn't enough to enable these services on my phone and that I needed this supposed IMS registration, the rep went back and forth with me and his help desk a few times, we reset the phone twice, and eventually it worked and showed in the phone information menu (*#*#4636#*#*)

IMS registration status: available, VoLTE: available

WiFi calling - when I hit the toggle this time just moved to on and stayed on, didn't even have to do the setup process (presumably because it had been done several times before).

VoLTE - verified by making a call and phone stayed on LTE, did not drop to "H"SPA as it had been doing before.

verified wifi calling by putting phone into airplane mode, enabling wifi and the network in the notification shade said "Telus Wifi Calling"

The guess is non-carrier (Telus in this case) purchased phones are not whitelisted or in the system (their IMEI?) to automatically get IMS registered without tech support assistance.

I think in every 3rd party (Google in this case) purchase phone, the user WILL need to call in to tech support and speak to an agent familiar with the issue or is encouraged to really dig into the things they need to do on their end to get it working.

I also wonder if the device model has to be an approved one by Telus. e.g. can previous Pixels which have not been officially listed by Telus get this working if they support the features? Will only device models Telus has sold themselves work? I don't have answers to these questions and other users will have to try. I'd recommend obscuring what your phone model is or telling Telus it's one of the supported devices to get an honest attempt from them at getting working rather than just repeating a script that it's not supported. We want the real answer whether it can be done or not.

bonus round - Telus does not have eSIM beyond Apple watch at this time, but is working on it for other devices for future support (Pixel 3 supports it)

Telus does not have RCS/Chat (enhanced/advanced messaging) at this point, but is working on it for future availability

Also discovered sometime in the last few years is that Telus permits MMS usage whilst still maintaining a data block on your account, so you can be free to receive (and send if you have a package) picture and video messages, and not worry about pay-per-use data if you dont have a package for that. (it used to be that a data block blocked MMS too, it was all or nothing)

I'm disappointed this information was so hard to find on the net and wasn't provided by any of the providers themselves. There should be information on Telus' VoLTE and WiFi calling pages, for example that explicitly state users with 3rd party purchased devices will need to call in to tech support to get their device activated. I'm sad for mainstream users that A. dont know about these features existing B. don't know they need to be added to their account for billing purposes C don't know they're not actually working at that point and that they D need to call in and work with a tech to get them actually functional

I hope these posts help others who come after me who had the same questions and issues I did.