iPhone unavailable

After 10 years of using Android, I switched last year to Apple and a nightmare problem has appeared. I carry my phone in my pocket all the time. I carefully switch off the display before doing so. Despite this, the phone seems to come alive while in my pocket (this never happens on Android) and after a while locks me out with the message "iPhone unavailable for 15 minutes". And yes, literally I am stuck holding this over-expensive brick in my hands for 15 minutes. I checked Apple Support and they made it worse by telling me next time the phone could lock me out permanently and I would have to erase the phone to recover it! What?? I am searching for a way to disable this stupid security feature. A thief may come once in a blue moon, but this brainless self-destruct could happen any time, any day, any week. How can I disable this iPhone Unavailable "feature"?