Here is your answer. The calls you receive are dialed through what's called an automated dialer. The "callers" do not know if you are answering the phone or if its a machine until the hear a beep in their ear. Only then can the "caller" tell the computer that you are not home and and he machine picked up. Because the callers are not calling from but on behalf of company's or organizations, callers are instructed to not leave messages. Callers working an outbound center have no control over who is called, what time they are called or how often you are called. I would encourage those who have complained to be happy these individuals are still emotes as this type of job are the ones slowly being shipped over seas.
Try putting yourself in the callers position who is simply doing their job, that includes having people yell at them, call them names, loudly slam the phone and curse at them.