Since people seem to be perfectly willing to pay a bundle of money to a real estate agent, or in this case, to yourself, going out for estimates may be part off the territory. Probably, the customer is not trying to get 6 low ball estimates on a 1/2 day job. So to answer the question, some plumbers will quote a flat rate on the phone for "X" like replace water heater, replace PRV, install toilet. etc. I get the impression from the posts that most of our guys will not go out to your house to esimtate these types of jobs. Want to remodel a bathroom....that is worth a quote.
Let's here what the others have to say.