In “Step by Step Guide – How to Create a Voice Booking Assistant – Cal.com & Google Cal in Retell AI,” Henryk Brzozowski walks you through building a voice AI assistant for appointment booking in just a few clicks, showing how to set up Retell AI and Cal.com, customize voices and prompts, and automate scheduling so customers can book without manual effort. The friendly walkthrough makes it easy to follow even if you’re new to voice automation.
The video is organized with clear steps and timestamps—copying the assistant, configuring prompts and voice, Cal.com setup, copying keys into Retell, and testing via typing—plus tips for advanced setups and a preview of an upcoming bootcamp. This guide is perfect if you’re a beginner or a business owner wanting to streamline customer interactions and learn practical automation techniques.
Project Overview and Goals
You are building a voice booking assistant that accepts spoken requests, checks real-time availability, and schedules appointments with minimal human handoff. The assistant is designed to reduce friction for people booking services by letting them speak naturally, while ensuring bookings are accurate, conflict-free, and confirmed through the channel you choose. Your goal is to automate routine scheduling so your team spends less time on phone-tag and manual calendar coordination.
Define the voice booking assistant’s purpose and target users
Your assistant’s purpose is to capture appointment intents, verify availability, create calendar events, and confirm details to the caller. Target users include small business owners, service providers, clinic or salon managers, and developers experimenting with voice automation. You should also design the assistant to serve end customers who prefer voice interactions — callers who want a quick, conversational way to book a service without navigating a web form.
Outline core capabilities: booking, rescheduling, cancellations, confirmations
Core capabilities you will implement include booking new appointments, rescheduling existing bookings, cancelling appointments, and sending confirmations (voice during the call plus optionally SMS/email). The assistant should perform availability checks, present available times, capture required customer details, create or update events in the calendar, and read a concise confirmation back to the user. Each capability should include clear user-facing language and backend safeguards to avoid double bookings.
Set success metrics: booking completion rate, call duration, accuracy
You will measure success by booking completion rate (percentage of calls that result in a confirmed appointment), average call duration (time to successful booking), and booking accuracy (correct capture of date/time, service, and contact details). Track secondary metrics like abandonment rate, number of clarification turns, and error rate for API failures. These metrics will guide iterations to prompts, flow design, and integration robustness.
Clarify scope for this guide: Cal.com for scheduling, Google Calendar for availability, Retell AI for voice automation
This guide focuses on using Cal.com as the scheduling layer, Google Calendar as the authoritative availability and event store, and Retell AI as the voice automation and orchestration engine. You will learn how to wire these three systems together, handle webhooks and API calls, and design voice prompts to capture and confirm booking details. Telephony options and advanced production concerns are mentioned, but the core walkthrough centers on Cal.com + Google Calendar + Retell AI.
Prerequisites and Accounts Needed
You’ll need a few accounts and basic tooling before you begin so integrations and testing go smoothly.
List required accounts: Cal.com account, Google account with Google Calendar API enabled, Retell AI account
Create or have access to a Cal.com account to host booking pages and event types, a Google account for Google Calendar with API access enabled, and a Retell AI account to build and run the voice assistant. These accounts are central: Cal.com for scheduling rules, Google Calendar for free/busy and event storage, and Retell AI for prompt-driven voice interactions.
Software and tools: code editor, ngrok (for local webhook testing), optional Twilio account for telephony
You should have a code editor for any development or script work, and ngrok or another tunneling tool to test webhooks locally. If you plan to put the assistant on the public phone network, get an optional Twilio account (or other SIP/PSTN provider) for inbound/outbound voice. Postman or an HTTP client is useful for testing APIs manually.
Permissions and roles: admin access to Cal.com and Google Cloud project, API key permissions
Ensure you have admin-level access to the Cal.com organization and the Google Cloud project (or the ability to create OAuth credentials/service accounts). The Retell AI account should allow secure storage of API keys. You will need permissions to create API keys, webhooks, OAuth clients, and to manage calendar access.
Basic technical knowledge assumed: APIs, webhooks, OAuth, environment variables
This guide assumes you understand REST APIs and JSON, webhooks and how they’re delivered, OAuth 2.0 basics for delegated access, and how to store or reference environment variables securely. Familiarity with debugging network requests and reading server logs will speed up setup and troubleshooting.
Tools and Technologies Used
Each component has a role in the end-to-end flow; understanding them helps you design predictable behavior.
Retell AI: voice assistant creation, prompt engine, voice customization
Retell AI is the orchestrator for voice interactions. You will author intent prompts, control conversation flow, configure callback actions for API calls, and choose or customize the assistant voice. Retell provides testing modes (text and voice) and secure storage for API keys, making it ideal for rapid iteration on dialog and behavior.
Cal.com: open scheduling platform for booking pages and availability management
Cal.com is your scheduling engine where you define event types, durations, buffer times, and team availability. It provides booking pages and APIs/webhooks to create or update bookings. Cal.com is flexible and integrates well with external calendar systems like Google Calendar through sync or webhooks.
Google Calendar API: storing and retrieving events, free/busy queries
Google Calendar acts as the source of truth for availability and event data. The API enables you to read free/busy windows, create events, update or delete events, and manage reminders. You will use free/busy queries to avoid conflicts and create events when bookings are confirmed.
Telephony options: Twilio or SIP provider for PSTN calls, or WebRTC for browser voice
For phone calls, you can connect to the PSTN using Twilio or another SIP provider; Twilio is common because it offers programmable voice, recording, and DTMF features. If you want browser-based voice, use WebRTC so clients can interact directly in the browser. Choose the telephony layer that matches your deployment needs and compliance requirements.
Utilities: ngrok for local webhook tunnels, Postman for API testing
ngrok is invaluable for exposing local development servers to the internet so Cal.com or Google can post webhooks to your local machine. Postman or similar API tools help you test endpoints and simulate webhook payloads. Keep logs and sample payloads handy to debug during integration.
Planning the Voice Booking Flow
Before coding, map out the conversation and all possible paths so your assistant handles real-world variability.
Map the conversation: greeting, intent detection, slot collection, confirmation, follow-ups
Start with a friendly greeting and immediate intent detection (booking, rescheduling, cancelling, or asking about availability). Then move to slot collection: gather service type, date/time, timezone and user contact details. After slots are filled, run availability checks, propose options if needed, and then confirm the booking. Finally provide next steps such as sending a confirmation message and closing the call politely.
Identify required slots: name, email or phone, service type, date and time, timezone
Decide which information is mandatory versus optional. At minimum, capture the user’s name and a contact method (phone or email), the service or event type, the requested date and preferred time window, and their timezone if it can differ from your organization. Knowing these slots up front helps you design concise prompts and validation checks.
Handle edge cases: double bookings, unavailable times, ambiguous dates, cancellations
Plan behavior for double bookings (reject or propose alternatives), unavailable times (offer next available slots), ambiguous dates (ask clarifying questions), and cancellations or reschedules (verify identity and look up the existing booking). Build clear fallback paths so the assistant can gracefully recover rather than getting stuck.
Decide on UX: voice-only, voice + SMS/email confirmations, DTMF support for phone menus
Choose whether the assistant will operate voice-only or use hybrid confirmations via SMS/email. If callers are on the phone network, decide if you’ll use DTMF for quick menu choices (press 1 to confirm) or fully voice-driven confirmations. Hybrid approaches (voice during call, SMS confirmation) generally improve reliability and user satisfaction.
Setting Up Cal.com
Cal.com will be your event configuration and booking surface; set it up carefully.
Create an account and set up your organization and team if needed
Sign up for Cal.com and create your organization. If you have multiple service providers or team members, configure the team and assign availability or booking permissions to individuals. This organization structure maps to how events and calendars are managed.
Create booking event types with durations, buffer times and availability rules
Define event types in Cal.com for each service you offer. Configure duration, padding/buffer before and after appointments, booking windows (how far in advance people can book), and cancellation rules. These settings ensure the assistant proposes valid times that match your operational constraints.
Configure availability windows and time zone settings for services
Set availability per team member or service, including recurring availability windows and specific days off. Configure time zone defaults and allow bookings across time zones if you serve remote customers. Correct timezone handling prevents confusion and double-booking across regions.
Enable webhooks or API access to allow external scheduling actions
Turn on Cal.com webhooks or API access so external systems can be notified when bookings are created, updated, or canceled. Webhooks let Retell receive booking notifications, and APIs let Retell or your backend create bookings programmatically if you prefer control outside the public booking page.
Test booking page manually to confirm event creation and notifications work
Before automating, test the booking page manually: create bookings, reschedule, and cancel to confirm events appear in Cal.com and propagate to Google Calendar. Verify that notifications and reminders work as you expect so you can reproduce the same behavior from the voice assistant.
Integrating Google Calendar
Google Calendar is where you check availability and store events, so integration must be robust.
Create a Google Cloud project and enable Google Calendar API
Create a Google Cloud project and enable the Google Calendar API within that project. This gives you the ability to create OAuth credentials or service account keys and to monitor API usage and quotas. Properly provisioning the project prevents authorization surprises later.
Set up OAuth 2.0 credentials or service account depending on app architecture
Choose OAuth 2.0 if you need user-level access (each team member connects their calendar). Choose a service account if you manage calendars centrally or use a shared calendar for bookings. Configure credentials accordingly and securely store client IDs, secrets, or service account JSON.
Define scopes required (calendar.events, calendar.freebusy) and consent screen
Request minimal scopes required for operation: calendar.events for creating and modifying events and calendar.freebusy for availability checks. Configure a consent screen that accurately describes why you need calendar access; this is important if you use OAuth for multi-user access.
Implement calendar free/busy checks to prevent conflicts when booking
Before finalizing a booking, call the calendar.freebusy endpoint to check for conflicts across relevant calendars. Use the returned busy windows to propose available slots or to reject a user’s requested time. Free/busy checks are your primary defense against double bookings.
Sync Cal.com events with Google Calendar and verify event details and reminders
Ensure Cal.com is configured to create events in Google Calendar or that your backend syncs Cal.com events into Google Calendar. Verify that event details such as title, attendees, location, and reminders are set correctly and that timezones are preserved. Test edge cases like daylight savings transitions and multi-day events.
Setting Up Retell AI
Retell AI is where you design the conversational brain and connect to your APIs.
Create or sign into your Retell AI account and explore assistant templates
Sign in to Retell AI and explore available assistant templates to find a booking assistant starter. Templates accelerate development because they include basic intents and prompts you can customize. Create a new assistant based on a template for this project.
Copy the assistant template used in the video to create a starting assistant
If the video demonstrates a specific assistant template, copy or replicate it in your Retell account as a starting point. Using a known template reduces friction and ensures you have baseline intents and callbacks set up to adapt for Cal.com and Google Calendar.
Understand Retell’s structure: prompts, intents, callbacks, voice settings
Familiarize yourself with Retell’s components: prompts (what the assistant says), intents (how you classify user goals), callbacks or actions (server/API calls to create or modify bookings), and voice settings (tone, speed, and voice selection). Knowing how these parts interact enables you to design smooth flows and reliable API interactions.
Configure environment variables and API keys storage inside Retell
Store API keys and credentials securely in Retell’s environment/settings area rather than hard-coding them into prompts. Add Cal.com API keys, Google service account JSON or OAuth tokens, and any telephony credentials as environment variables so callbacks can use them securely.
Familiarize with Retell testing tools (typing mode and voice mode)
Use Retell’s testing tools to iterate quickly: typing mode lets you step through dialogs without audio, and voice mode lets you test the actual speech synthesis and recognition. Test both happy paths and error scenarios so prompts handle real conversational nuances.
Connecting Cal.com and Retell AI (API Keys)
Once accounts are configured, wire them together with API keys and webhooks.
Generate API key from Cal.com or create an integration with OAuth if required
In Cal.com, generate an API key or set up an OAuth integration depending on your security model. An API key is often sufficient for server-to-server calls, while OAuth is preferable when multiple user calendars are involved.
Copy Cal.com API key into Retell AI secure settings as described in the video
Add the Cal.com API key into Retell’s secure environment settings so your assistant can authenticate API requests to create or modify bookings. Confirm the key is scoped appropriately and doesn’t expose more privileges than necessary.
Add Google Calendar credentials to Retell: service account JSON or OAuth tokens
Upload service account JSON or store OAuth tokens in Retell so your callbacks can call Google Calendar APIs. If you use OAuth, implement token refresh logic or use Retell’s built-in mechanisms for secure token handling.
Set up and verify webhooks: configure Cal.com to notify Retell or vice versa
Decide which system will notify the other via webhooks. Typically, Cal.com will post webhook events to your backend or to Retell when bookings change. Configure webhook endpoints and verify them with test events, and use ngrok to receive webhooks locally during development.
Test API connectivity and validate responses for booking creation endpoints
Manually test the API flow: have Retell call Cal.com or your backend to create a booking, then check Google Calendar for the created event. Validate response payloads, check for error codes, and ensure retry logic or error handling is in place for transient failures.
Designing Prompts and Conversation Scripts
Prompt design determines user experience; craft them to be clear, concise and forgiving.
Write clear intent prompts for booking, rescheduling, cancelling and confirming
Create distinct intent prompts that cover phrasing variations users might say (e.g., “I want to book”, “Change my appointment”, “Cancel my session”). Use sample utterances to train intent detection and make prompts explicit so the assistant reliably recognizes user goals.
Create slot prompts to capture date, time, service, name, and contact info
Design slot prompts that guide users to provide necessary details: ask for the date first or accept natural language (e.g., “next Tuesday morning”). Validate each slot as it’s captured and echo back what the assistant heard to confirm correctness before moving on.
Implement fallback and clarification prompts for ambiguous or missing info
Include fallback prompts that ask clarifying questions when slots are ambiguous: for example, if a user says “afternoon,” ask for a preferred time range. Keep clarifications short and give examples to reduce back-and-forth. Limit retries before handing off to a human or offering alternative channels.
Include confirmation and summary prompts to validate captured details
Before creating the booking, summarize the appointment details and ask for explicit confirmation: “I have you for a 45-minute haircut on Tuesday, May 12 at 2:00 PM in the Pacific timezone. Should I book that?” Use a final confirmation step to reduce mistakes.
Design polite closures and next steps (email/SMS confirmation, calendar invite)
End the conversation with a polite closure and tell the user what to expect next, such as “You’ll receive an email confirmation and a calendar invite shortly.” If you send SMS or email, include details and cancellation/reschedule instructions. Offer to send the appointment details to an alternate contact method if needed.
Conclusion
You’ve planned, configured, and connected the pieces needed to run a voice booking assistant; now finalize and iterate.
Recap the step-by-step path from planning to deploying a voice booking assistant
You began by defining goals and metrics, prepared accounts and tools, planned the conversational flow, set up Cal.com and Google Calendar, built the agent in Retell AI, connected APIs and webhooks, and designed robust prompts. Each step reduces risk and helps you deliver a reliable booking experience.
Highlight next steps: implement a minimal viable assistant, test, then iterate
Start with a minimal viable assistant that handles basic bookings and confirmations. Test extensively with real users and synthetic edge cases, measure your success metrics, and iterate on prompts, error handling, and integration robustness. Add rescheduling and cancellation flows after the booking flow is stable.
Encourage joining the bootcamp or community for deeper help and collaboration
If you want more guided instruction or community feedback, seek out workshops, bootcamps, or active developer communities focused on voice AI and calendar integrations. Collaboration accelerates learning and helps you discover best practices for scaling a production assistant.
Provide checklist for launch readiness: testing, security, monitoring and user feedback collection
Before launch, verify the following checklist: automated and manual testing passed for happy and edge flows, secure storage of API keys and credentials, webhook retry and error handling in place, monitoring/logging for call success and failures, privacy and data retention policies defined, and a plan to collect user feedback for improvements. With that in place, you’re ready to deploy a helpful and reliable voice booking assistant.
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