>> However the nature of the intention is simply to open the app and push the appropriate button,
which they can absolutely do, because you capture their phone number from them when they _install_ the app (ie run it for the first time.)
In other words;
a) visit web page, sign up for service, install PWA or app or whatever, enter config details...
... time passes.....
b) ahh - Panic - open app - press button....
>> The user may not have the time or the skills to enter a login code, and then push the option button for fire ambulance or police.
I'm guessing they probably also don't have time to visit the site, download the app, install it, whatever either. So your flow suggests that step a) above happens long before panic time...
Cheers
Bruce