An emergency click nobody answers is a purchase made for a competitor
Unplanned plumbing demand goes to whoever picks up, which makes the phone a paid media setting rather than an operations detail.
Somebody standing in water is not leaving a message and is not waiting for a callback. They are working down the results until a person answers. You paid to be first in that sequence and then handed the job to whoever was second.
That makes the honest question before any budget conversation a simple one. During the hours you intend to advertise, who answers, how quickly, and what happens on the calls that come in while they are already on a call.
It also means advertising hours should follow answering hours rather than demand. Running ads at two in the morning because that is when pipes burst, with nobody awake, is buying clicks for the company that does answer.
An answering service can work, and the standard is higher than most owners assume. If the service cannot take enough detail to be useful, cannot say anything about when somebody can come, and does not reach a human on your side quickly, it produces a record of calls rather than jobs.
This is worth testing rather than assuming. Call your own advertised number at ten at night, from a number nobody recognizes, and take note of what happens. It is a five minute exercise that occasionally changes a budget decision.