On my iphone, can I have callers that are not in my phonebook automatically go to voicemail?

Hi, I've had an iphone 3G for about 3 months (AT&T) and love it, I never expected to find a handheld device become so indispensable..But I'm going to try to not digress here... My dilemma is this - I spend a lot of time on call. My work pager is not so reliable so I give out my cell number as a backup. Customers take note of my cell number and call me back 3 months later at 3 in the morning on a week that I'm not on call, this is bad. Is there any way that I can setup my phone so that any number calling me that is NOT in my contacts simply goes to my voicemail? Then I can leave my phone on at night in case one of my kids needs to reach me (for instance), but I don't need to worry about a customer calling me in the middle of the night and waking me up only for me to refer them to the person who is on call? I would simply remove the setting and "open" the phone up for weeks that I'm on call... I can't seem to find a way to do this, any help or suggestions would be appreciated. Thanks!