For instance, the app could say something like, "Please wait." or "I'll be right back". A friend asked me if it was possible, and since I don't have an iPhone I couldn't answer the question.
Thanks.
No, that would require the third party app to be active while the phone app was active, which is not currently possible. Third-party (developer created) apps are open when they are open and then quit when some other app is open.