Alarms - that is why I have two alarms and one has a battery back up in it.
I did have trouble with my bluetooth connecting to my car. All is well now. I had to delete all saved connections on the phone and car, then start over.
My other half's alarm on iPhone 6 has failed to wake him up only once (and I think it was for a weird setup reason).
But we do have a pretty reliable backup - one of the cats comes to meow and demand for attention a few minutes before the alarm goes off.
(Much better than the cat in Ireland - I'd wake her up, no matter if I'd go to early shift or late shift)