Simple iPhone app

I would like to develop a simple iphone app. The user of this app will create a text corresponding for his/her contacts (all or some of them). And if this number calls, it needs to display the text corresponding to the contact.   For example, in my app I go to “Pascal” which is in my iPhone contacts and which is also a friend but not that close to me and I write “pascal’s wife is pregnant”. If pascal calls, it needs to display in the screen “pascal’s wife is pregnant”.   As I am not an expert, I don’t know if Apple allows this kind of control.