Douglas,
I can't understand how you can filter people without first speaking to them to find out what they want. In my experience people are very short, sweet and to the point with their answering machine messages, so if you don't call them back how do you find out what they want and what the job entails. Presumably you filter out jobs that are too small or too far.
To be perfectly honest, I think you're doing a spot of trumpet blowing, whilst throwing your professional credibility to the four winds. It's just plain rude to ask people to leave a message and then because you don't like the tone of their voice, or whatever other filtration criteria you can glean from a short message, not get back to them.
Simon.