Change voice during phone call android

Question

I want to make an android application that allow user change the voice during phone call. For example: You are a man, you can change the voice to a woman or robot when talking o

Answer 1

The app you linked isn't changing voices on the phone: it uses SIP (or similar) to place a call through the authors' servers and the voice changing happens there. That's why you only get a small number of free minutes of use before you have to pay them.