AI Voice Agent ROI: How to Calculate Your Payback Period

AI voice agents are easy to sell on vibes: "24/7 coverage," "never miss a call," "capture more revenue." But the actual purchase decision for a field service business comes down to one question: when do I get my money back?

This post walks through the exact ROI calculation — the inputs, the math, and worked examples for different business types. If you're evaluating an AI voice agent, this is the framework.

The Core ROI Formula

AI voice agent ROI for inbound call capture comes down to four variables:

• Calls missed per week (after-hours + overflow during business hours)
• Capture rate (what % of those missed calls the AI will handle successfully)
• Average ticket value (what a converted job is worth)
• Conversion rate (what % of captured calls become booked jobs)

The formula:

Finding Your Inputs

Calls Missed Per Week

Most field service businesses underestimate this. To find the real number, check your voicemail count for a typical week, look at missed call logs if your phone system tracks them, or review after-hours call volume specifically (most phone systems can pull this by time of day).

For a business open 8am-5pm, Monday-Friday: after-hours covers 73% of the week by hours. That's evenings, nights, weekends, and holidays. An HVAC company that gets 10 calls per day during business hours typically gets 3-6 after-hours calls per day it's missing entirely.

Rule of thumb if you don't have data: take your average inbound calls per business hour, multiply by 1.5, that's a conservative estimate of after-hours daily volume.

Capture Rate

A well-configured AI voice agent captures 85-95% of calls that reach it. "Capture" means: the caller doesn't hang up before the AI qualifies them and captures the job details.

Variables that affect capture rate: call type (emergency vs. non-urgent), AI agent quality, first response latency, and how natural the conversation sounds. Well-implemented systems running on platforms like Retell AI consistently see 90%+ capture rates.

Use 85% as a conservative estimate in your model.

Conversion Rate

What % of captured inbound calls become booked jobs? For after-hours calls, this is typically higher than daytime calls because:

• Emergency calls have high urgency — the customer needs service now
• Callers who reach your AI rather than going to voicemail are more likely to stay with you
• AI booking systems eliminate the scheduling friction that costs daytime conversions

Conservative estimate: 55%. Realistic for established businesses: 65-75%.

Average Ticket Value

Use your actual average ticket value, not your largest job. For inbound emergency calls specifically, the average tends to be higher than walk-in or outbound quote requests because emergency services command premium pricing.

Worked Examples

Example 1: HVAC Company (Mid-Size)

Example 2: Plumbing Company (Smaller)

Example 3: Landscaping Company

System Cost Variables

AI voice agent systems for field service businesses typically cost:

• Implementation (one-time): $3,000-$12,000 depending on complexity, integrations, and call volume
• Monthly platform cost: $300-$1,500 depending on call volume and platforms used
• Voice AI platform (Retell/Vapi): $50-$300/month depending on call minutes

Total monthly operating cost for a typical mid-size field service business: $500-$1,500/month.

The Honest Caveats

These calculations assume the AI agent is properly configured for your business — not a generic voice bot. A misconfigured system with high hang-up rates or poor qualification will see much lower capture and conversion rates.

The calculations also assume you have the technician capacity to handle the additional booked jobs. If you're already booking out 3 weeks, capturing more calls improves your scheduling efficiency but doesn't immediately convert to the numbers above.

Finally: these are recovery numbers, not total revenue. Some of those calls would have left a voicemail and called back during business hours. The AI captures the incremental portion that would have walked.

Even with conservative adjustments for all of the above, the math for after-hours AI voice agents for field service businesses almost always shows a payback period under 30 days. That's why this is typically the first automation implementation we recommend.