All the time I served as a business advisor I not only returned calls but I harped on the subject with my clients. Whether a response is positive, negative or "we don't know yet," the caller deserves to hear the answer. Retuning the call just might stop those annoying calls. If you don't want to talk with someone call when you are pretty sure that you will get the answering machine or better yet have your secretary or assistant make the call for you. At least a message is delivered. It does not hurt you to return the call but someone might be hurting if you do not even if your response is negative.
Not returning calls is not only impolite, it is unprofessional!!
There, that is said. Enough already!