> ## Documentation Index
> Fetch the complete documentation index at: https://docs.unleashx.ai/llms.txt
> Use this file to discover all available pages before exploring further.
# Voice AI Agent Function calls
> How function calling lets your UnleashX voice agent take real actions mid-conversation — transfer calls, check and book calendar slots, and run your own custom functions.
# Voice AI Agent Function calls
> **In one line:** Function calls let your voice agent *do* things during a live call — not just talk. Mid-conversation, the agent can hand off to a human, look up availability, book a meeting, or hit your own API, then keep speaking naturally with the result.
## What is a function call?
A voice agent's core skill is conversation. But most useful tasks need it to reach outside the conversation — into a calendar, a CRM, a phone system, or your backend. A **function call** (also called *tool calling*) is how the agent does that.
You describe an action to the agent as a **function**: a name, a plain-English description of when to use it, and the **parameters** it needs. During a call, when the caller's intent matches that description, the agent:
1. **Decides** the function is needed based on what the caller said.
2. **Extracts** the parameters from the conversation (e.g. a date, a name, a phone number).
3. **Calls** the function — UnleashX runs it against the real service.
4. **Uses the result** — the agent speaks the outcome back to the caller and continues the conversation.
The key idea: *you* define the "what" (the function and its parameters), and the *agent* decides the "when" and fills in the "with what" from natural speech. No rigid menus or keypad trees.
Function calls run **in real time, mid-call**. The caller experiences it as a natural pause ("let me check that for you…") rather than a handoff to a form.
## How it works in a voice call
```
Caller speaks → Agent understands intent → Agent picks a function
↑ │
│ ▼
Agent speaks ← Agent phrases the result ← UnleashX runs the function
```
Because the whole loop happens during the call, latency matters. UnleashX streams the agent's speech, calls the function, and resumes speaking as soon as the result returns — so the caller hears a brief, natural "one moment" rather than dead air.
## Built-in function calls
UnleashX ships with ready-made functions for the most common voice-agent actions. Turn them on for an agent and it can use them immediately — no code required.
Hand the live call off to a human or another number when the caller asks for a person, hits an edge case, or needs escalation. The agent transfers the active call to a configured destination and can pass along context.
Look up real-time availability on a connected calendar. The caller asks "what's open Thursday afternoon?" and the agent reads back the free slots.
Create a booking on the connected calendar once the caller picks a time — capturing name, time, and any details, then confirming the appointment out loud.
Define your own function that calls your API or workflow. Give it a name, a description, and parameters, and the agent will invoke it whenever the conversation calls for it. See below.
## Custom function calls
When the built-ins aren't enough, define a **custom function** to connect the agent to any service you can reach over HTTP — your order system, a lookup service, a payment link generator, or an internal workflow.
A custom function is defined by three things:
| Part | What it is | Example |
| --------------- | --------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------- |
| **Name** | A short identifier for the action. | `lookup_order_status` |
| **Description** | Plain-English guidance on *when* the agent should call it. This is what the agent matches against. | "Use when the caller asks about the status of an existing order and gives an order number." |
| **Parameters** | The inputs the function needs, each with a type and description so the agent knows what to extract. | `order_number` (string), `email` (string, optional) |
At call time, UnleashX sends the extracted parameters to your endpoint, waits for the response, and hands the result back to the agent to speak.
Write the **description** like an instruction to a new teammate: say exactly when to use the function and when *not* to. A clear description is the single biggest factor in whether the agent calls the right function at the right moment.
## Setting up a function call
Sign in at [https://www.tryunleashx.com](https://www.tryunleashx.com) and open the voice agent you want to extend.
In the agent's **Function calls** section, enable a built-in (Transfer, Check slots, Book slots) or choose **Add custom function**.
For built-ins, connect the required service (e.g. a calendar or transfer destination). For a custom function, set the **name**, **description**, **parameters**, and the **endpoint** to call.
Run a test call and speak the trigger scenario. Confirm the agent calls the function, extracts the right parameters, and speaks the result naturally.
## Best practices
* **One job per function.** Narrow, single-purpose functions are easier for the agent to choose correctly than one function that does many things.
* **Describe the trigger, not the mechanism.** The description should say *when* to use the function in the caller's terms, not how your backend works.
* **Mark optional vs. required parameters.** If a parameter is required, the agent will ask the caller for it before calling — make sure that question feels natural.
* **Keep endpoints fast.** Since the caller is waiting on the line, aim for responses in a second or two; return a graceful message the agent can speak if something goes wrong.
* **Confirm irreversible actions.** For bookings, transfers, or anything that changes data, have the agent read back the details and confirm before it acts.
## Related
* [Making outgoing calls](/phone-calls/making-outgoing-calls) — how calls are placed and received.
* [MCP Integrations](/mcp/integrations) — connect calendars, CRMs, and other tools your functions can reach.
* [Agent setup](/playground/agent-setup) — configure and test your agent before going live.