Before a plumber comes out, ask: “What will I owe for this appointment if I don't approve any work?”
Then ask what that charge includes. “A fee to come out” might pay for the trip, finding the problem, some labor or a combination. Shops don't all use the same terms.
Trip or dispatch charge: Pays for the visit. Don't assume it also includes diagnosis or repair.
Diagnostic fee: Pays for investigating the problem. Finding why a heater stopped working is a service even if you choose another company for the repair.
Minimum charge: Sets the least you'll pay for the specified service or labor. A short job can still be billed at that minimum.
Ask the shop to explain its arrangement in dollars and included work. Knowing that one shop charges a trip fee and another has a minimum doesn't tell you which visit will cost less.
Pricing a replacement and finding the cause of a problem aren't necessarily the same service.
Fictional example: You ask one shop for a price to replace a water heater, and it offers a free estimate. On another call, you ask why the hot water keeps cutting out. The free replacement estimate doesn't tell you what the diagnostic visit costs.
Be clear about which appointment you're requesting. If the shop offers both, ask what each includes.
Some companies credit a visit fee toward work you approve. Others don't. Check the conditions rather than assuming the invoice will show a credit.
Also ask about evening, weekend and holiday charges before booking. A company's “emergency service” advertisement doesn't explain its pricing policy by itself. If the situation may be urgent, describe it and ask about the appropriate response rather than deciding to wait based on price alone.
Ask how the shop will price and request approval for work beyond the appointment. If the plumber needs more investigation, find out what that adds before agreeing.
Get the booking terms in a message or email. Paying for a diagnosis can make sense. Knowing what you're paying for is the part to settle before the visit.