I remember when I was having a family vacation in Bali a few years ago, I received a surprise call from my dad and he simply asked me how's my holiday and I said it was great. Only later I realised he saw the news about earthquake in Bali. I feel bad to know that while I was having a great time, he was in fact worrying the whole time.
Being parents, we don't have a radar to monitor our children's every move (they can decide to keep their phones in silent mode). But it is the children's responsibility to be contactable at all time, for parent's peace of mind.