> ## Documentation Index > Fetch the complete documentation index at: https://docs.retellai.com/llms.txt > Use this file to discover all available pages before exploring further. # Function calling for single and multi-prompt agents > Function calling lets Retell single or multi-prompt agents take real actions — transfer calls, end calls, book appointments, send SMS, and call external APIs. ## Introduction Function calling transforms your AI agent from a conversational interface into an action-oriented assistant. By connecting your agent to functions, you enable it to interact with external systems, manage call flows, and perform real-world tasks. ## Common use cases Function calling enables your agent to: ### Call management * **Transfer calls**: Route to human agents or other departments * **End calls**: Gracefully terminate conversations * **Send DTMF tones**: Navigate phone menus ### Business operations * **Book appointments**: Integrate with scheduling systems * **Check availability**: Query calendars or inventory * **Process orders**: Create, modify, or cancel orders ### Data integration * **Retrieve information**: Pull data from CRMs, databases, or APIs * **Update records**: Modify customer information or case details * **Send notifications**: Trigger emails, SMS, or push notifications ## How function calling works ### The process 1. **Agent Decision**: Based on the conversation context, the LLM determines when a function is needed 2. **Parameter Extraction**: The agent extracts required information from the conversation 3. **Function Execution**: Retell calls your function with the extracted parameters 4. **Response Handling**: The function result is processed and incorporated into the conversation ### Technical overview Function calling uses structured JSON to communicate between the LLM and your systems: ```json theme={"dark"} { "function": "book_appointment", "arguments": { "date": "2024-03-15", "time": "14:00", "customer_name": "John Smith" } } ``` ### Additional resources * [OpenAI's Function Calling Guide](https://platform.openai.com/docs/guides/function-calling): Technical deep dive * [Function Calling Tutorial](https://semaphoreci.com/blog/function-calling): Step-by-step implementation guide ## Configuring tool calls ### Dashboard configuration Access and manage tools in the agent detail page under the "Functions" section. Functions section in dashboard ## Available function types ### 1. Pre-built functions Retell provides ready-to-use functions for common scenarios: | Function | Purpose | Common Use Cases | | ------------------------------------------------------------- | --------------------------------- | -------------------------------------------- | | [**End Call**](/build/single-multi-prompt/end-call) | Terminate conversation gracefully | After completing tasks, when caller requests | | [**Transfer Call**](/build/single-multi-prompt/transfer-call) | Route to human agents | Escalation, department routing | | [**Press Digits**](/build/single-multi-prompt/press-digit) | Send DTMF tones | Navigate IVR systems, enter codes | | [**Send SMS**](/build/single-multi-prompt/send-sms) | Send text messages | Confirmations, follow-ups | For appointment scheduling, use the [Cal.com integration](/integrations/cal-com) or [Calendly integration](/integrations/calendly). The built-in **Check Availability** and **Book Calendar** tools are no longer offered here — see the [deprecation notice](/deprecation-notice/2026/10-31_legacy_calcom_tools) for what happens to agents that still have them. ### 2. Integration tools Call a connected provider (HubSpot, Salesforce, Zendesk, Cal.com, and the other [supported providers](/integrations/overview#supported-providers)) live during the conversation — look up the caller, book an appointment, create a ticket. Retell handles the provider's API and auth. See [integration tools](/build/single-multi-prompt/integration-tools) for setup. ### 3. Custom functions Create your own functions to: * **Integrate with your APIs**: Connect to CRM, ERP, or custom systems * **Execute business logic**: Validate data, calculate prices, check inventory * **Trigger workflows**: Start processes in other systems #### Custom function features * **Flexible parameters**: Define any input structure * **Async execution**: Run in background without blocking conversation * **Error handling**: Graceful fallbacks for failures * **Response control**: Customize what the agent says during/after execution [Learn more about custom functions →](/build/single-multi-prompt/custom-function) ### 4. Code tool Run JavaScript code directly in Retell's sandbox without setting up an external server. Useful for: * **Data transformation**: Format, combine, or compute values from dynamic variables * **Simple API calls**: Fetch data using the built-in `fetch()` function * **Logic and calculations**: Implement conditional logic, math, or string operations [Learn more about Code Tool →](/build/single-multi-prompt/code-tool) ## Video tutorial