Used to have my phone in my jacket pocket. Lost it once, then found it about an hour later.
now I keep it in a zipper pocket in my pants. No problems and most of the time I can get to it fast enough not to miss the call.
Zipplock bag are great I use them when going cliff jumping, but on the mountain if your gear can hold out against moisture you should be fine.
oh ya I also keep a paper towel or 2 in the same pocket, soaks up any moisture.
Soon Ill have to find a good pocket for an Ipod