How to do realtime audio mixing?

Question

linux

I want to play a friend as if they are calling him from the radio, there is a gsm-gateway, a left sim card and a familiar girl with a good voice. To make everything look like in reality, you can be able to mix jingles into the sound in real time (for example, applause, or a radio splash screen). But the question is how to do it? I played with the settings, I can either transfer the stream from the microphone to the inline line, or from the audio output. But at the same time - even it is impossible to do so.
(There are two operating systems - Windows 8 and Linux Mint, it doesn't matter for me where to work, the main thing is to make it work)

Answer 1

put the mobile phone next to the microphone, from which to turn on the "jingles", given that the quality of mobile communication leaves much to be desired, the difference between this option and "transfer the stream from the microphone or from the audio output to the inline line" no one will notice anyway.