Usecase: my hotline has phone number 0815/111222333 user starts up dialer and dials that number instead of calling my app starts up to offer him service.
I did it on Android, though I am pretty sure this won't be allowed in iOS. But I didn't find that explicit statement. I am just looking for confirmation.
It is most likely not possible on iOS. Event if it was I noted that there is a patent on this.