Make a pouch out of several layers of bubble wrap. Slip the phone in and slide the pouch into a dry sack. To have all the angles covered you could stuff in a spare battery, charger unit, Sidewinder manual power unit and auxiliary antenna if your phone can use one.

A dedicated Pelican box with cubed foam as padding would also work.

When selecting a phone for emergency use lean for a combination of reception and battery life. Stay away from features that consume excessive power like large color displays and cameras.

The ability to send text messages, I think most phones have this, is useful and can sometimes get through when a voice connection won't.