By starting with: I can give you an estimate over the phone. If I have to come there, that's a service call for troubleshooting.
I explain that a service call is $150 for the first hour, and $100 for each additional hour. (I might pro-rate if it comes up.)
Except for a price range phone estimates are worthless for the customer. I can almost guarantee some contractor will quote a rediculously low price over the phone just to get the customer to have them come out. When he gets there he gives them the real price and it's a lot higher than the phone estimate.
I had a lady call me and wanted a price to relocate some electrical in a wall she was tearing out. I told her there would be a small dispatch fee for me to come out and give an estimate for the work and if she approved the work while I was there the dispatch fee would be waived.
She didn't want to pay a dispatch fee for me to come out and insisted on me giving her some kind of idea on how much it would cost. I told her it sounds like it would be at least $200. She then told me the reason she wanted to know was because another electrician told her it would be $100 over the phone but when he got out there he wanted more than $200 to do the work. She refused to have him do the work and sent him away without paying him anything for coming out because he told her he would come out for free and give her an estimate.
I have a hard enough time getting people to pay me a small dispatch fee of say $29 to come out and give them an estimate when there are many contractors in my area willing to do it for free. If I tried to charge $150 to go out and give estimates I don't think I would get any takers.
I have never been able to win quoting prices over the phone. They just keep calling until they find someone who quotes a lower price.