But Google's answer is not the whole answer. I refer you to my last sentence. An SMS message works on a lower standard of signal and the reception does not need to last very long for the message to pass. However, your phone has to be registered so do it now folks, don't wait until you need it.
OK, but I just see it as a complication when you are in an emergency situation, according to your link, you need to text 999 with the following information :
Which service you require
Description of the problem
Exactly where you are
This last is a highlighted issue (by several emergency services) in that you need to get your W3W App and find your location and remember it (exactly), go onto your text screen / app and then include those 3 words in your text.
I'm not particularly adept at texting (all stiff fingers and thumbs) and being Diabetic the circulation in my fingers is the first to go in cold weather.
The problem highlighted is that if your W3W are (say) "Understanding : Horse : Rainbows" but when you come to type it out you remember (say) "Understand : Horses : Rainbow", those odd few missing / extra letters will mean you are not where you say you are.
It is a free choice, I just personally see too many possibilities for errors in time of pain and stress.
Each to their own.