you'd have to use WIFI to access/connect to chromecast. That is, both chromecast and your phone will have to be on the same network.
Once you started to stream, chromecast will pull video (if you're watching video) directly from internet (thru your home wifi and your broadband) without going thru your phone. that means, you can actually use your iphone anyway you want while it's streaming including turning off its wifi if you need to. it's very neat.