manipulating the voice in a mobile phone

Hi everybody,

Can anyone helps me on how to manipulate the my voice that is coming out of the mobile phone while having a conversation with a friend.
For Example:
I want to build an application and download it in a smart phone or any other mobile phone. The purpose of this application is to get my voice while talking to a freind and change, do anything with it, before it goes to the other end, my friend. The type of the change is not the issue, the issue is how to intercept my voice and change it.
If somebody has a source, example, or article please help..