I work in a call center.
One day I heard from across the office a girl who got really upset to the point of tears and then I saw the supervisor pick up the phone and shortly after he hung up. I thought nothing of it. Probably just a really rude customer. The next day a different supervisor call me over to listen to the call.
Everything sounded normal until the agent came back from a brief hold to speak with the customer and all you could hear were growls. It didn't sound like growls that can be produced by a human. Sounded like a demon straight out of a horror movie. The agent kept trying to get a response and the growls started to sound angrier and louder. It sounded as if the more the agent spoke the more agitated the "creature" got. This continued for about 5 minutes until the girl freaked out and had her supervisor take the call. The supervisor said he was going to disconnect the call and he called her back. The customer picked up the call and spoke is if nothing happened.