> ## Documentation Index
> Fetch the complete documentation index at: https://documentation.uponai.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Set and Parse Custom SIP Headers

> Pass custom data between systems using SIP headers on inbound calls, outbound calls, and call transfers.

Custom SIP headers are used in telephony to pass information between systems. This feature is available for phone calls only. UponAI supports:

* **Receiving** custom SIP headers on inbound calls
* **Sending** custom SIP headers on outbound calls
* **Sending** custom SIP headers when initiating call transfers

<Warning>
  Many SIP parsers and middleboxes assume a maximum of 1024 bytes per header line and may reject or truncate longer headers if not explicitly configured to allow more.
</Warning>

## Parse Custom SIP Headers for Inbound Calls

Custom SIP headers (headers starting with `X-` or `x-`) on inbound calls are automatically received, extracted, and available in `call.custom_sip_headers`. They are also automatically added to your dynamic variables — accessible in your agent prompt by stripping the `X-`/`x-` prefix.

No configuration is required for inbound SIP header parsing.

**Example:** if an inbound call carries these SIP headers:

```json theme={null}
{
  "X-test-header": "something",
  "X-another-header": "something else",
  "from": "1234567890",
  "to": "0987654321"
}
```

Then `call.custom_sip_headers` will contain — and these will be added as dynamic variables:

```json theme={null}
{
  "test-header": "something",
  "another-header": "something else"
}
```

<Note>
  If you have already set a dynamic variable with the same name for the call, it will override the value received from the SIP header.
</Note>

## Set Custom SIP Headers for Outbound Calls

Add custom SIP headers to outbound calls as needed. All custom header names must start with `X-`.

## Set Custom SIP Headers for Call Transfers

You can also set custom SIP headers on call transfers. Header names must start with `X-`. Dynamic variable values are supported, so you can pass information extracted during the call to the receiving party.

<Warning>
  For cold transfers using a transferee number, UponAI uses SIP REFER. Some telephony providers — including Twilio — do not honor custom SIP headers in REFER requests. If you are using Twilio, custom SIP headers set in the transfer call tool will not be delivered on cold transfers.
</Warning>

For implementation details:

* **Conversation Flow agents** — see [Call Transfer Node](/build/conversation-flow/call-transfer-node)
* **Single / Multi Prompt agents** — see [Transfer Call](/build/transfer-call)