Assume you are calling a customer care center from your telephone. Once connected, you hear an automated message saying that all of their customer care executives are busy, and so ask you to wait on the line. You may even be one of many waiting for the same service executive (concurrency and multiple threads). Now, you have two options to choose from; one is to hold the line and wait for someone to speak to, and two is to hang up and try again after some time. However, trying again after some time may have you ending up in the same situation.
How about the customer care executive calls you back? You dial the number, the system picks up your number, and says that it will call you back automatically when the customer care ...
