You can't get the phone via any easy API within iOS. There might be a way to do it via the technique described in this related question, but this is a few years old and Apple may have closed this hole (which uses an undocumented key). I also wonder if Apple wouldn't allow the app on the App Store for privacy reasons. Probably not.
You'll have to trust the user to type in the correct phone number if you prompt him/her to.