Just thought I'd share something that works well for us.

We make the phone field on our forms optional. If someone does leave a phone number, a text message alert is sent to the cell phone of girl we use in Colorado who calls them back within the hour. Her goal is to catch them on our site, tell them about our site's cool features and feel them out. If they are a "hot" lead, she marks it in our database and assigns a task to have our buyers agent follow up further. It works great. For $3/call, we don't have to waste time following up with people who aren't interested in us following up.