You can waste a lot of time on estimates. I try to use my experience and gut to judge if it is worthwhile. I also try to engage them in conversation as to why they want to replace those parts. If they are having problems I will speculate some about causes. What they usually want a price on is wrong at least 75% of the time unless they are price shopping some other shop's estimate. I often succeed in getting them to come in without giving a phone estimate. I had a guy this week who wanted an estimate on an ignition switch. When I engaged him in conversation it didn't sound like an ignition switch at all. I persuaded him to bring it to us and we wound up selling him a starter. If it is something like brake pads, oil change, alignment, tires, etc. I will give a price. If it is work for maintenance such as a tuneup, timing belt, flushes, etc. I will work up a quote. Often in those cases I am competing with the dealer.