It is 8:14 on a Tuesday night. A homeowner's water heater just failed and the basement is filling. They Google a phone answering service the way most people do — they call the first plumber on the list. That plumber's line rings four times and drops to voicemail. They do not leave a message. They call the next number. By 8:19 they have a company booked for 7 a.m., and it is not you.
You never saw the call. You will not see it in the morning either, because there is no voicemail to return. From your side, nothing happened. From the customer's side, you did not exist. That is the quiet way an after-hours answering service gap costs a service business real revenue — one unremarkable evening at a time.