In reply to David Coley:
I had my phone blown out of my hand whilst taking a photo on Blencathra back in the winter. It mainly landed in snow, but a bit hit a rock that fortunately only crack the screen protector, not the screen itself. I use a rubberised lightly 'armoured' case on mine which I'm sure helped a bit. So although I've done it very rarely I know it is possible to drop your phone when climbing, particularly if you use it as your primary camera.
Last weekend I went to Gogarth (see other thread!) and had the foresight to take off the phone cover, drill two small holes in the bottom of the case and thread a piece of thin but strong nylon cord through it. I tied that into a loop and attached it to a neck loop (made of paracord or similar - I have a number of these from down the years, as I always put my car key on one and tuck that down inside my base layer when winter climbing). This worked really well as on the hanging belays of Wen Zawn you are quite conscious that if you drop anything you won't be getting it back!
So you might already have all the stuff you need for a DIY solution.