Optus can barely make a regular 4G call at this point. The drop in call quality when routing between networks is absurd. The last 000 call we made was so bad that we had dropouts whist standing in the middle of a suburban footy oval.
I've also found that it's possible to completely break VoWifi so it will say it's connected but fail to make or receive calls. Just enable 464XLAT on wifi and connect a Google Pixel to it.
The Optus network is about 15 years behind the competition at this point.