Can't find out if daemon application can be run on non-jailbroken iPhone. And what is Apple policy about this stuff? I don't really know if my idea is possible at all... I wonder if it's possible to create such a daemon that can intercept outcoming user call and play custom tone signal while remote user does not answer our call. Almost all cell-operators provides a service of custom call tones which means you can listen to some music while waiting for response... This drives me wild.
Plain and simple: It's not allowed.
Taken from the official App Store Review Guidelines
2.8 Apps that install or launch other executable code will be rejected