I make it really simple. Every install has two phases, the design process and the estimating & construction process. The second cannot happen without the first, and a s a design/consulting firm we charge for that phase. You have to know your market and know what the quantity of leads you can expect is, though, because obviously this slows the flow. On the positive side, you're only seeing qualified, serious buyers at this point. Someone "calling around for estimates" isn't going to hop on board.
You need to know what you're selling and how you're selling it, and recognize that if you hit resistance on the phone you're highly unlikely to convince them of the value of paid design services at the appointment.
I guess a good thing with being in the DC market is the fact that the people in gov't work understand bidding. I've had people argue with me that they send out RFPs all day long and get loads of responses, to which I ask "but have you set all the specifications, and everyone is bidding the exact same thing?" The good ones go "ohhhhhhh I get it" and the others get told thanks so much for calling but I don't think we're a fit for what you're looking for.