Seriously, if you're worried about water and your phone use aquapac case or keep your smartphone at home, take cheap payg instead.
Yes, that's what I do. Little £20 Nokia 100, switched off and in the bottom of the bag like any other bit of emergency equipment. The problem with a lot of smartphones is that the battery life is fairly poor - most of us are in the habit of charging them on a daily basis. It's for this reason that I take an Ipod Touch for reading in the tent. It would be most embarrassing to exhaust a smartphone battery by reading or watching a film and then being unable to make an urgent call.
you find two dead Deer on one of your walks the smell is bad but the bacteria is even worse how would you have known about the Deer if you had camped even a few hundred meters down stream out of site and smell. you had decided to have a nice cold drink while setting up camp and putting tea on next day you continue your walk but in a different direction to the dead Deer with another overnight camp. Just before you stop to set up camp one or both of you start to feel really ill and you have a two day ( in good health) walk out.
Honestly not worth thinking about for the cost of a filter between the two of you.
To be honest, I can't think of many worse places than the outdoors to get an attack of the two bob bits with optional vomiting (a spacesuit is one of them). You'll run through (as it were) your small stock of bog roll in no time...
My Drinksafe filter weighs about 100g and takes up less space than a compact camera so it's a no-brainer for me but I know there's guys on here who've drunk unfiltered water for years with (allegedly) no ill effects.