Not that I can think of any use for that app, but wouldn't it be more convincing if you played some audio that sounds like the fake caller talking through the speaker? With some pause in between to answer/comment. That would make it much more convincing. Maybe you can record the audio on your own or find something on freesound.org. Bonus points for an option to select both male and female voices. Extra bonus points for different choices of talk (calm, loud, shouting, hectic, ...).