https://w3w.me.ss/ collects examples of bad experiences with W3W. It is linked in the paper.
I think the issue is that other less emergency numbers can't (like 101). That's why they tell you to hang up and call 999 rather than transferring you.
But I don't know why they just don't add more numbers to the "allowed to geolocate you" list. Or even build in a "send my location" button in phone calls. You wouldn't even need to modify the phone system - it could send DTFM tones.
https://www.mirror.co.uk/tech/google-now-automatically-provi...
But the W3W app identifies your position by GPS, meaning it is subject to the same general reliability problem via inheritance. Yes, it displays a map as well, so you can identify that you need to quote a different square, but in an emergency situation I'd posit that people are more likely to just read off the words and not really take in the displayed map. And the same solution works for GPS alone anyway – if you are reading your position using a phone app it is likely displaying an approximate map also.
You will need reporter to describe location.