Uhm i don't know... i don't think it would work they way you described. let me explain: I, user, will not buy your app because it can updates my number because

Uhm i don't know... i don't think it would work they way you described. let me explain: I, user, will not buy your app because it can updates my number because it might never happen. As you said it's rare that people change number so i would buy your app just to see it lay there. I might buy to send the notification, maybe. in the end it's probably just an email and i do not care if they will update the contact manually or via an app. A possible solution would be, make it free, and let people buy a "change number token" via in-app purchase. I am still not sure it might take off but this way you have more possibility to open your user base.