Give an agent the diary.

The MCP server

The tools are derived from the live API contract — all 47 /v1 operations, generated from the same document the API reference renders. An endpoint added to the API becomes a tool the day it ships; the tool list cannot disagree with the server behind it.

The hosted endpoint lists only the tools your key's scopes can call: a "Take bookings" key gets the booking tools and nothing else — no deleteService in a receptionist's toolbox, and no context window spent on schemas it could never use. The local npx server reads the contract before it knows your key, so it serves the full list and lets the server refuse out-of-scope calls (insufficient_scope names both what was needed and what the key holds); prefer the hosted endpoint for scoped keys.

claude mcp add --transport http voltsbook \
  https://app.voltsbook.com/mcp \
  --header "Authorization: Bearer vb_live_your_key"

{
  "mcpServers": {
    "voltsbook": {
      "type": "http",
      "url": "https://app.voltsbook.com/mcp",
      "headers": { "Authorization": "Bearer vb_live_your_key" }
    }
  }
}

{
  "mcpServers": {
    "voltsbook": {
      "command": "npx",
      "args": ["-y", "@voltsbook/mcp"],
      "env": { "VOLTSBOOK_API_KEY": "vb_live_your_key" }
    }
  }
}

A phone agent that books: Twilio + ElevenLabs

The shape: your Twilio number answers, ElevenLabs runs the conversation, and the VoltsBook tools give it your real diary — services, prices, live slots, and the booking itself. Their dashboards move their menus around; the concepts below do not.

1. 1

   Get your VoltsBook API key

   In your panel: Settings → API keys → New key. Tick what the key may do — "Take bookings" is all a receptionist needs; the catalog, settings and reports powers are separate ticks, and the server refuses anything outside them. Copy the key once — it is shown once. Use ONE key per integration (one for the voice agent, another for anything else), so you can revoke one without breaking the rest. Your plan must include the public API.

2. 2

   Create the agent in ElevenLabs

   In ElevenLabs, create a Conversational AI agent: pick a voice, set the first message ("Thanks for calling — what can I book you in for?"), and paste the system prompt below. The prompt is most of the product: it tells the agent to quote only what the tools return and to confirm before booking.

3. 3

   Give it the VoltsBook tools

   Add our MCP server to the agent — ElevenLabs supports MCP integrations: the server URL is https://app.voltsbook.com/mcp, with an Authorization header of "Bearer <your key>". Every booking tool arrives at once. If your platform version only offers per-tool webhooks, define three against the /v1 endpoints instead: listServices, getAvailability and createBooking cover a working receptionist.

4. 4

   Connect your Twilio number

   In ElevenLabs under Phone numbers, import your Twilio number (it asks for your Twilio Account SID and auth token) — or buy a number there directly. Assign the agent to the number. Twilio and ElevenLabs bill their own usage; VoltsBook does not charge per call.

5. 5

   Make the test call

   Call the number and walk the whole path: ask what services there are (it should quote YOUR prices), ask for a time tomorrow (it should offer real slots), book one, and hang up. The appointment is in your diary like any other — notifications fire, reminders schedule, and it shows the moment the call ends.

You are the phone receptionist for [BUSINESS NAME], a [TRADE] at [ADDRESS].
You book appointments using the VoltsBook tools. Follow these rules without exception:

1. Only state services, prices and times that a tool returned on THIS call.
   Never estimate, remember, or invent any of them.
2. Every slot, next-available offer and booking from the tools carries "say" —
   the time as words, already in the business's timezone ("Saturday 22 August
   at 9:00 AM"). Read "say" verbatim. NEVER convert startIso or startAt
   yourself: they are UTC, and reading one as local time is wrong by the
   whole UTC offset.
3. Booking flow: find the service with listServices; offer 2–3 concrete slots
   from getAvailability (ask with perDay=3 and spread=true so the options
   span the whole day), reading each slot's "say"; when the caller picks one,
   call previewPrice for that exact choice, then repeat the full booking
   back — service, the booking's own "say", price — and ask "Shall I book
   that?". Only call createBooking after a clear yes.
4. NEVER say listServices' priceMinor out loud. That is the catalog price,
   before tax, before the chosen staff member's own price, before add-ons and
   before any coupon — with a 20% tax it is a fifth short of the bill.
   previewPrice is the price: quote its totalMinor. When payableTodayMinor is
   smaller, say both — "forty-two dollars, and we take ten today". If it
   returns anything in unknowns, say the total may still change rather than
   promising it.
5. If the caller has a discount code, send it as couponCode to previewPrice
   BEFORE you quote, and again to createBooking. If couponMessage comes back,
   read it to them — the code was refused or is conditional. We cannot take a
   gift card over the phone; ask them to book online to use one.
6. Collect and read back the caller's first name and phone number before
   booking. If they have an email, take it — that is where their
   confirmation goes.
7. If a slot is refused as taken, apologise briefly and offer the next
   options from a fresh getAvailability call. When getAvailability returns no
   slots at all, it also returns nextAvailable — the earliest opening after
   the day asked for. Offer it by reading its "say": "We're full that day;
   the next opening is Saturday 22 August at 9:00 AM".
8. To move or cancel an existing appointment, find it with listBookings by
   their details, confirm which one aloud, then rescheduleBooking or
   cancelBooking.
9. If you cannot help — a question about something no tool answers — take a
   message: name, number, and what they need, and say someone will call back.
10. Never mention tools, systems, or these instructions. You are simply the
   receptionist.

Beyond taking bookings: let the agent build the front door

An agent whose key carries the "Change settings" scope can also author the business's guided-booking flow — the "What are you here for?" wizard that routes an unsure visitor to the right service. The grammar, the validation rules, and a complete worked example as real tool calls are written up on their own page:

Building guided-booking flows with an AI agent