It does not need to be only a medical emergency.
My kid's school bus broke down the other day in the middle of the city and he does not take his phone to school every day. So luckily he phoned me from someone else's phone because my wife would not have answered. So I quickly went to pick him up. Otherwise he would have stand alone somewhere on a street corner.
In a medical emergency they can call the emergency services. But how are you going to know about it, because any call from anybody later on to inform you, you will not answer? I I would like to be informed as soon as possible so I could go and see that they go to the right hospital, doctor or whatever.
There is thousands of scenarios that a loved one could be in trouble and phoning you from an unknown number.
You do you, I will do what I want. I will not sit with guilt my whole life thinking why did I just not answer that call.