### Answer The native Cal.com integration in Retell AI supports custom fields for booking appointments. [(1) ]() When you enable "collect phone number - required" in Cal.com, you need to instruct your agent to collect that information and pass it through the booking function. The changelog states: > "If you've added custom fields in Cal.com, you can now use them in Retell. When using Cal.com functions, you can instruct the agent to collect specific information, and it will automatically display the collected data in the booking event." Steps to resolve: 1. Keep the phone number field enabled as required in Cal.com 2. Update your agent's prompt to explicitly collect the phone number before calling the booking function 3. Ensure your prompt instructs the agent to pass the collected phone number to the

book_appointment

function For conversation flow agents, you may want to add a conversation node before the booking function to collect and confirm the phone number, then pass it along. [(2) ]() If you're still experiencing issues, ensure your Retell and Cal.com timezones are aligned, and that your prompt makes the LLM aware of the current time. [(3) ]() Check out these potentially useful sources for more details: - [Book Calendar]() - [Platform Changelogs | Retell AI]() - [Dynamic Variables]() Would you like me to try to provide a general answer on how to structure the prompt for collecting custom fields?

​ To learn more, see the sources I considered: [[1] Platform Changelogs | Retell AI](https://www.retellai.com/changelog) [[2] cal.com check availability and book appointment function](https://discord.com/channels/1186424376300679209/1438984963008757841) [[3] Issue with book_appointment (Cal.com Native Integration) for Round Robin & Custom Location Event Typ](https://discord.com/channels/1186424376300679209/1379631392312266893) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer Based on the available sources, there is a video tutorial that demonstrates how to use Cal.com custom fields (including phone number) with Retell AI's booking function. [(1) ]() In the video, the workflow shown is: 1. Set up the booking questions (including phone number) in Cal.com's advanced page 2. Configure the agent to collect name, email address, and phone number before booking 3. Use the preset

check_calendar

and

book_appointment

functions with your Cal.com API key and event type ID The video demonstrates a successful booking where the phone number is collected and correctly appears in the Cal.com booking event. Troubleshooting suggestions: - Ensure your prompt explicitly instructs the agent to collect the phone number before calling the

book_appointment

function - Verify your Retell and Cal.com timezones are aligned [(2) ]() - Make sure your prompt makes the LLM aware of the current time using the

{{current_time}}

dynamic variable If the issue persists specifically when the phone number field is required in Cal.com, this may indicate a configuration or compatibility issue that isn't fully documented in the available sources. I'd recommend reaching out for additional support: - Visit [Contact Sales]() - Check [) Would you like me to try to provide a general answer on potential workarounds?

​ To learn more, see the sources I considered: [[1] Retell AI - How to use Cal.com Custom Field]() [[2] Issue with book_appointment (Cal.com Native Integration) for Round Robin & Custom Location Event Typ](https://discord.com/channels/1186424376300679209/1379631392312266893) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer Based on your logs, the LLM is extracting the parameters but not including the phone number field in the function call arguments. This is a prompt engineering issue—you need to be more explicit about when and how to pass the phone number. Recommended prompt structure: In your prompt, be very explicit about the function call requirements. For example: [(1) ]()

3. Confirm the date, time, and timezone selected by user: "Just to confirm, you want to book the appointment at ...". Make sure this is a time from the available slots.
4. Once confirmed, call function book_appointment to book the appointment.

Adapt this pattern to explicitly include phone number collection: - Instruct the agent to collect the phone number before confirming the booking - Explicitly state that the phone number must be included when calling the

book_appointment

function - Consider adding a confirmation step: "Just to confirm, I have your name as John, email as xxx@hotmail.com, and phone number as +1234567890. Is that correct?" Example prompt addition:

When user selected a slot, please book the appointment by calling the `book_appointment` function.

[(2) ]() Expand this to something like: "Before calling the

book_appointment

function, you MUST collect and confirm the customer's name, email, AND phone number. Include ALL three fields when calling the function." Additional troubleshooting: Make sure your prompt includes the current time reference using

{{current_time}}

in your prompt—this helps the LLM understand temporal context for booking. [(3) ]() The available sources don't provide specific documentation on the exact parameter names for Cal.com custom fields within the booking function. You may need to verify the exact field name expected (e.g.,

phone

,

phone_number

,

attendee_phone

) in your Cal.com event type configuration. If the issue persists after strengthening your prompts, I'd recommend reaching out for additional support: - Visit [Contact Sales]() - Visit [)

​ To learn more, see the sources I considered: [[1] Build a multi-prompt agent](https://docs.retellai.com/build/single-multi-prompt/write-multi-prompt) [[2] Book Calendar](https://docs.retellai.com/build/book-calendar) [[3] Issue with book_appointment function](https://discord.com/channels/1186424376300679209/1341725205445476413) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.