'I don't have time to talk right now'

You'll recognize this common response from a prospect who wants to get off the phone. My response is to say, "No problem; when is the best time to reach you for a three-minute conversation. I want to see if we're even a good fit for you before we talk at any length..."

This way you don't sound desperate and you let them know you only need a few minutes. More often than not they stay on the phone with you once they realize it won't be long.