I'm on Bell Mobility in Ontario and can report the same issue with Iphone 11 pro.. horrible sound quality with the phone's radio. Actually the worst sounding calls I've heard on a mobile phone in a long time. Tried earpiece, speakerphone, I've swapped out to a new SIM, tried disabling/enabling VoLTE, even tried wifi calling over my LAN, makes no difference. I've used Skype & Whatsapp to make calls instead of the phone app and it's a lot better but there is still a slightly artificial, robotic, sound to the calls on both ends.
I'm pretty disappointed because otherwise, it's a nice phone: very fast gui, great battery life, audio quality when listening to music is excellent on the phone's speakers, with wired headphones, or bluetooth, which you'd think would make phone calls sound good also but no. I've given up. Using the same SIM in an old Samsung S7 or a new Pixel 4 XL, phone call sound quality is much, much better. No comparison. This is my first iphone. I wanted to see what the fuss was about. It's pretty good for anything except making or getting phone calls. :-) For my situation, it's unusable.