Default [Q] App plays soundfile during phoning to dialog partner

Hello there!

I have a principle question!

Is it possible for an App, during I am phoning, playing a short mp3 (eg "Happy Birthday to you"). That means for my dialog partner it would sound, like I said to him that words. I myself, do not hear that sound, the App directly should put the sound internal to the "micro".

In other words, is it possible with an App to access/influence the "Telephon functionality"? (Or is the "Telephon functionality" a black box without accessibility?)

Thanks in advance,