Forget calling people. Open a store. Have an online booking system. When someone comes in after booking a time, have staff with some basic training go through

Forget calling people. Open a store. Have an online booking system. When someone comes in after booking a time, have staff with some basic training go through a list of things to get a rough diagnosis. Give the customer an estimated repair time and cost. Simple things should be replaced on the same day. That's how you do it.