Pebble will probably release an emulator for it at some stage to help developers get software out there before the actual watches arrive, but it may not be for a while yet. You could always email the developer support people and ask them about it.
My app can text people, but only via an HTTP to SMS gateway. If you want to make calls or send texts from the phone itself, I suspect you'll need to write an app for the phone, and on the iPhone I think this will require user confirmation. Other people may be better placed to advise here as I've not done any phone app development.