Voice & SMS
How we built an AI receptionist that books jobs from a live calendar and warm-transfers emergencies, with the demo
It's 7pm and a pipe has burst under someone's kitchen sink. They ring the first plumber on the list. The owner is on another job with his hands full, so the call goes to voicemail. The caller hangs up and rings the next company.
Invoca, a call analytics company, found that under 3% of home services callers who reach voicemail leave a message. For an emergency, that number is the job.
We built an AI receptionist to handle exactly that call. It books routine jobs from the live calendar. When it hears an emergency, it puts the caller through to the on-call tech, after telling the tech who is calling and why. I recorded a full demo of it, which is below.
Watch the demo
The video shows three live calls: a routine booking, a booking where the first time the caller asked for was full, and an emergency that gets warm-transferred. Then it walks through the n8n workflows and data behind each one.
What's in the video:
- 0:00 Intro
- 0:34 How it fits together: the n8n gateway
- 1:21 Live call 1: a routine booking
- 3:37 The results: workflow runs, the booking, HubSpot and alerts
- 6:22 Live call 2: a double booking and a change of mind
The emergency call and its warm transfer come after that.
The short answer
The agent answers every call, books routine jobs into real open slots, and warm-transfers emergencies to a person with a private briefing. If nobody picks up, it takes the details and sends an urgent alert. Every call ends up in the CRM with a summary.
| Question | Short answer |
|---|---|
| What it does | Answers, books from live availability, warm-transfers emergencies, logs every caller to HubSpot |
| What it doesn't do | Quote prices, or handle the emergency itself once a person has it |
| Who it suits | Plumbing, HVAC, electrical, roofing, cleaning, or any trade that books visits by phone |
| Built with | Retell for the voice, GPT-4.1 mini, n8n, HubSpot, Google Calendar, Google Sheets and Twilio |
| Full write-up | The case study, with screenshots of every workflow |
How one call flows through it
The voice agent does the talking and nothing else. Behind it, an n8n gateway checks each request, loads that business's settings and hands the work to small services for the calendar, the CRM and alerts.
Warm transfer, explained
There are three ways an AI agent can hand a call to a person, and Retell's transfer documentation describes all three.
In a cold transfer, the call goes straight to the other number and the agent drops off. The person who answers knows nothing about the caller.
In a warm transfer, the agent stays on the line. It can wait until a real person answers, then play a whisper message that only the person hears. It can also introduce both sides in a short three-way message.
In an agentic warm transfer, a second AI agent talks to the person first and decides whether to connect the caller or cancel.
We use a warm transfer. When a caller describes a burst pipe, a gas smell or no heat in winter, Ava tells them she is connecting them. The on-call tech hears a private whisper with the caller's name, address and problem before the call connects. The caller never has to repeat themselves.
Retell lets you set how long the other phone rings before the transfer counts as unanswered. We set ours to 25 seconds. If nobody picks up in that time, the agent goes back to the caller, takes the details and sends an urgent alert straight away.
Booking from real availability
Most voice bots can talk about appointments. Fewer can see the calendar. Before this agent offers a time, it checks working hours, travel time between jobs, minimum notice and the ZIP codes the business serves. If the time the caller wants is taken, it offers the nearest open slots and confirms the one they pick.
Every booking goes into one ledger, which is the single source of truth. If the caller changes their mind on the same call, the booking is moved, not duplicated. Our post on automatic appointment booking covers why that matters.
The CRM and the call log
Callers are matched by phone number, so a repeat caller updates their existing contact instead of creating a new one. The agent writes the name, address, ZIP, lead status and a short summary to HubSpot. A small link table stops a second call creating a duplicate before HubSpot's search has caught up.
When the call ends, a post-call workflow waits for Retell's finished analysis. It then saves the summary, transcript, recording link, outcome and urgency to a call log and to the contact. The owner can see what happened on any call without listening to it.
One build, any business
The business name, services, service area, hours, emergency keywords, transfer number, calendar, CRM and alert channels all sit in one config row. The agent reads that row at the start of every call. Adding a second business means adding a second row and pointing a phone number at it. The prompt and the workflows don't change.
That is also what makes it swappable. The CRM, calendar and alert services each take the same input and return the same result. There's a ready Google Sheets adapter and a template for other CRMs, so moving from HubSpot to something else doesn't touch the agent.
What it costs to run
Retell bills by the minute. Its own guide to warm transfers says pay-as-you-go pricing starts from $0.07 a minute. It also says a warm transfer is charged at the per-minute rate for the time the AI is on the call, with no separate transfer fee. That is a vendor's statement, and the real rate depends on the voice, the model and the phone line you choose.
Our post on AI receptionist cost prices a full month of calls on Retell and on six ready-made services, so you can compare a build like this with buying a plan.
What a person still handles
Emergency calls once they're transferred. The agent briefs the tech, and the tech takes it from there.
Quotes and pricing. The agent takes the details and passes them to the office instead of naming a price.
Callbacks on messages taken when a transfer wasn't answered.
When this isn't the right build
If you get a handful of calls a week and can call people back within the hour, a missed call text back is cheaper. If you don't have a shared calendar or job system, the agent has nothing to book into, so fix that first. And if most of your calls are long sales conversations, the agent can take details and book a visit, but the sale is still yours.
See it for yourself
Watch the video above, read the case study for screenshots of every workflow, or call our live demo and try to trip it up. Our AI for trades service covers builds like this for HVAC, plumbing and electrical firms, and our AI receptionist for plumbers page covers the plumbing version.
Questions people ask
What is a warm transfer in an AI receptionist? The AI stays on the line while it calls a person, waits for a real human to answer, and briefs them privately before connecting the caller. In a cold transfer the AI just passes the call on and drops off, so the person starts with no context.
What happens if nobody answers the transfer? In this build the on-call number rings for 25 seconds. If nobody picks up, the agent goes back to the caller, takes their name, address and problem, and sends an urgent alert straight away, so the job isn't lost.
Can an AI receptionist book into my real calendar? Yes, if it is connected to it. This agent checks working hours, travel time between jobs, minimum notice and your service area before offering a slot, then writes the booking to one ledger so nothing is double-booked.
Does the AI receptionist give prices? Not in this build. Pricing questions are passed to the office with the caller's details, because a wrong quote on the phone costs more than a short wait for the right one.
What tools is it built with? Retell runs the voice and the conversation on GPT-4.1 mini. n8n handles the gateway, booking logic and logging. HubSpot is the CRM, with Google Calendar, Google Sheets and Twilio behind it. The CRM, calendar and alert services can each be swapped.
Can the same build work for another business? Yes. Everything specific to a business sits in one config row: name, services, service area, hours, emergency keywords, transfer number and tools. A second business gets its own row and phone number, and the prompt and workflows stay the same.
Find out whether your calls justify a build like this.
Bring a week of your call log to the free audit. We count the calls, the emergencies and the ones that went to voicemail, and tell you whether a missed call text back, a ready-made plan or a build like this is the right step. If a cheaper option covers it, we say so.
Book the free audit →Sources
- Retell docs, transfer call tool for single and multi-prompt agents, checked November 2026
- Retell, warm transfer meaning: how it works and when to use it, checked November 2026
- Invoca, how much missed sales calls cost home services businesses, 2024