In my case I do not want people to know my real cell number so I use other apps like ringto or a similar app that gives me a phone number other than my real cell number, your app has an option to change the number however when sending a file or text my real number shows up and if that is what your app does then why should I use your app?
If my native app can do the same.
Giving the user the ability to show what ever number he or she chooses will give your app a great boost but on the mean time your app is no better than the rest.