jwolthuis, that's not the customer's fault, that's your's :).
What you need to do is qualify the customer. If they don't give you the right buying signals up front, unload them. You may have to spend some time determing what questions to ask to nail that down.
If someone's wasting my time on the phone, I wrap it up fast. "thanks for your time, goodbye. click" is what they get from me.
But buying customers? they love my service - they can call and get answers. Even for internet businesses, having great phone service can make sales, and solidify customer relations. I've got testimonials about this type of stuff on my website - e.g. called and spoke to a real human being.