What you are about to build

A phone number that answers itself. A call comes in, an AI agent picks up, has a normal conversation, collects the details, checks your calendar for a free slot, and books the appointment before the caller hangs up. You need a Vapi account (Vapi runs the voice agent), an n8n account (n8n writes the booking to your calendar), a calendar, and a phone number. Vapi bills by the minute of call time, so check its current pricing first.

The four moving parts

The assistant. The brain. A block of written instructions plus a language model that follows them. Almost all of the quality lives here. A good agent and a bad one are usually the same software with different instructions.

The number and the voice. How people reach it and what it sounds like. Get a number inside Vapi or bring your own, attach the assistant to it, and pick a voice. Judge a voice by how clear it is on a bad phone line, not by how good it sounds through your laptop.

The booking action. The agent cannot touch your calendar by itself. So you give it exactly one thing it may do in the real world: send the booking details somewhere. It fires once it has everything it needs, then waits for an answer before speaking again.

The calendar workflow. The n8n side, where the booking happens. It takes the details, checks the calendar for a free slot, creates the event, sends the confirmation, and replies back so the agent can say the time out loud. You plug an n8n calendar booking workflow in here rather than building one.

For the clicking, use Vapi's own documentation. It stays current so this page does not have to. All of this works the same whether your calendar is Google Calendar, Cal.com or Calendly. Only the last part changes.

The five decisions that make it good or bad

  1. What it may answer. Write the short list of questions it can handle and make everything else a callback. The bad version invents a price, and now you are honouring it.
  2. What it must collect before it books. Name, number, what they want, when. All four, confirmed back, before the action fires. The bad version books "Dave, Tuesday" and you cannot reach Dave.
  3. What it says when the slot is taken. Offer the next two open times and keep going. The bad version apologises, goes quiet, and leaves the caller to suggest something.
  4. When it hands off. Anyone asking for a person, sounding annoyed, or saying urgent gets a name taken and a callback promised. The bad version keeps trying to book them.
  5. Its first sentence. Short enough that nobody talks over it. The bad version is a three line greeting, the caller starts talking at line two, and the agent mishears them.

Your agent prompt, ready to paste

This goes in the assistant's system prompt, the standing instruction it reads before every call. Change every line marked \[EDIT THIS\] and leave the rest.

You are the phone assistant for [EDIT THIS: business name]. Be warm, brief, and useful.

Facts:
Hours: [EDIT THIS]
Time zone: [EDIT THIS: e.g. America/New_York]
What people can book: [EDIT THIS]
Questions you may answer: [EDIT THIS: one question and answer per line]

How you speak:
Short sentences. One question at a time, then wait.
Say dates plainly: "Tuesday the fourth, at two in the afternoon."
Repeat phone numbers back one digit at a time.
If the caller interrupts, stop and listen. If you did not hear something, ask them to repeat it.

On a booking call:
1. Ask how you can help.
2. Collect one at a time: full name, phone number, what they want to book, preferred day and time.
3. Read all four back and ask if that is right.
4. Once they confirm, say you are checking the calendar and use the book_appointment tool.
5. Wait for its answer. If booked, say the day and time again. If taken, offer two alternatives and try again.

Rules you do not break:
Never invent an available time. You only know a slot is free when the tool says so.
Never quote a price or timeline unless listed above.
For anything else, offer a callback and confirm the number.
If the caller asks for a person, sounds annoyed, or says urgent, stop booking. Take their name and number, end politely.
Never ask for card or bank details.
If asked whether you are a person, say you are an assistant and offer a callback.

What usually goes wrong