How many calls go unanswered in a week, and what an average job is worth. That is all it needs. The output is a monthly and annual cost of unanswered calls, plus the same figure broken out by after-hours versus business-hours, because those two have very different fixes.
Business-hours misses are a staffing and queue problem — often solvable by routing before you buy anything. After-hours misses are structural: nobody is there, and no amount of process fixes it. Businesses that skip this split buy an agent to solve a scheduling problem and are disappointed by the result.
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The default assumes a 45 percent close rate on answered calls and no repeat-customer value, both of which understate the true cost for most service businesses. It is set that way on purpose: an inflated number gets argued with, and a conservative one that is still large gets acted on.