Many many many years ago, I wrote an android app that allowed you to set the ringtone of the device you were ringing for that call (yes they had to have the app installed etc etc, and had given you permission to do it). I did release it but the market problem I had was that iPhone's ringtones were not something a client app could change, let alone in the background and even less so without direct permission of the user.

So anyway, if you want to do something like that, it's back to android I'm afraid. Don't worry, there's no patent and there's prior art (i.e. my usage). FWIW - the closest patent to it was made by a TV manufacturer that would allow you to find the remote control by playing a noise etc directed by the TV !