We've all heard the sound boards where they use a famous person's voice to prank people. My question is how do they do it?

I know i can just hold my phone up to the speakers but how can i hear the other party. I think a speaker phone would not be that great either. It seems like their ought to be a way to use the computer phone modem to do it? No?

Or somehow to output the soundcard into the jack on a cordless phone? What am i missing?