Voice Changer that can modulate the phone user's voice while live streaming in a phone call on their phone device.
Preferably it would just make the call through the user's mobile/cellular provider but if this is not possible we would need to look into a VOIP setup.
There is plenty of Voice modulation script that is public with abilities to change the users voice.
Eg. Kid, woman, old man, deep voice Etc.
Using this will reduce the coding you need to do substantially.
We need 5 free voices and 5 the user gets through making an in app purchase
Here is an example of the kind of app: https://play.google.com/store/apps/details?id=com.weirdvoice&hl
Something very similar to that