I have a mobile phone sized see-through case, exactly the same as buying one for an Ipad or similar. Bought it from Amazon, costs £5. On wet days I can even work my phone without taking it out of the case. I just use a thin lanyard/line which attaches to my shoulder strap and to the phone case and it then justs rests in my left side waist rucksack pocket. I have an Osprey Exel 48L. I've tried securing it to the shoulder strap with a 'D' ring (is that what its called?) but it restricts my phone movements too much, I had to bend my head to the phone rather than the phone being lifted up to my face. Just an ease of comfort thing! My system also works well for charging my phone on the move as well. I just put my phone battery charger in right-hand waist rucksack pocket and connect the charging lead from the charger in one pocket to the phone in the other pocket. I'm always careful if the wire across my waist is showing if I'm near the public and will always dismantle it. I've had some funny looks, you know, those kind of looks which none verbally asks if the wire is 'attached to something dangerous or not ' looks.
There can be a problem if you don't want your phone in one of these sealed containers (I prefer this unless it's wet) and because modern phones no longer have a corner area where a security lead can be attached I had to think of another method. To solve this little problem I also bought, again from Amazon, about £2.99 I think and it looks like a thick elastic band which grips the phone and again is attached to my lanyard/line.
In both cases it also ensures that when I bend over my phone does not smash to the ground.