Less than 15%. Most of the clients who call me already know they want us to take care of their trees - we just need to offer a reasonable price. Several clients just tell me what they want done and don't even ask for price - we've done work for them in the past and they trust that we will treat them fair.
I do try to sort them out with good questions on the phone. Don't do large removals, so those get kicked out right away. I ALWAYS thank them for calling even though we aren't going to help with their project - I'm never disappointed the phone is ringing with a potential customer on the other end.
I do charge a diagnostic fee, but not (yet) an estimate fee. There are times that can be a gray line...I try to be friendly to the client about that, but when I tell them on the phone "If I ran around looking at every tree we got a call about without charging, I wouldn't be able to feed my family" most people are very understanding. Some say they'll talk it over with their spouse. Maybe 30% of those folks call back...
If you just never show up...I suspect your calls will start to drop of quickly as word spreads. Tell them you are overbooked. Charge an estimate fee. Schedule initial appointments for 3 months out. But don;'t just not show up!