iPhone 11 - All Contacts Blocked

I have never seen it either but two relatives with it suggested a common factor like an installed app.

It is possible that

Settings, General, transfer or reset iphone, Reset, Reset all settings

will delete them but it also messes up the home screens, ringtones, mail accounts, preferences etc and for once I am not going to test it on my phone!

With a simple Auntie's phone setup it may be worth a try but back the phone up first in case the mess is too big. Getting the phone back as it was would need to restore to factory and restore from backup on setup. That may take longer than 1000 taps on the phone app.

"....One; HOW / WHY does this happen?..."

Did they install a security app or call ID checking app of some sort? It should not happen spontaneously.

"...Two; Is there an easier way ..."

Unfortunately not.