This has to be handled by your router and has nothing to do with the Arduino if it already works on the local network.
What you're looking for is called port forwarding. Any guide for setting up a home web server with your router would suffice.
The way I read it, the OP didn't have the router out in the field, and wanted just the iphone and Arduino to communicate. Thus, either both the iphone and the arduino have to support ad-hoc wifi networking without a base station, or something (presumably the iphone) has to act as the base station. If the iphone acts as a router, then it can route internet packets for the Arduino as well as just doing normal local communications.