### Answer --- The built-in

book_appointment

function for Cal.com doesn't automatically pass the phone number through. [(1) ]() You have two options to resolve this: Option 1: Update your prompt Add explicit instructions in your system prompt to always include the phone number when calling the booking function. For example: "Important: Always transfer the date/time, email, name and telephone number to the functions with each call." [(2) ]() Option 2: Build a custom function If you need more control over what data is sent, you can create a custom function that sends the phone number and any other information you want to your calendar. [(1) ]() This would involve setting up your own logic to send API calls to Cal.com. [(3) ]() If the phone number field isn't strictly required for your use case, another workaround mentioned is to disable the phone number requirement on your Cal.com event type. [(1) ]()

​ To learn more, see the sources I considered: [[1] Parsing Phone Number in Cal.com Book Appointment preset function](https://discord.com/channels/1186424376300679209/1438596252089716736) [[2] book_appointment doesn’t send phone attribute to cal](https://discord.com/channels/1186424376300679209/1329010496765956206) [[3] Integrate any system with custom function](https://docs.retellai.com/build/single-multi-prompt/custom-function) ​

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