Best way to be contactable for work while in water?

I'm on call for work and tend to surf in most of my spare time. I need a way of being contactable in the water. I know you can now get waterproof cases for iphones but after looking at them I would not trust my expensive phone in one being knocked about in the surf. I've seen a few "tough" phones on ebay but looks like most are just water resistant… For drops in the pool ect not extended aquatic use. Any cheap solutions? Plastic bag on old phone:-)

This is where maybe the apple watch may be a benefit when that comes out