> ## Documentation Index > Fetch the complete documentation index at: https://www.bolna.ai/docs/llms.txt > Use this file to discover all available pages before exploring further. # SIP headers on SIP trunk calls > Read P-Asserted-Identity and Diversion from inbound SIP trunk calls as prompt variables, and send your own SIP headers on outbound calls and transfers. SIP headers carry information alongside a call. On a SIP trunk you can read two carrier headers on inbound calls, and send headers of your own on outbound calls and on transfers. | Direction | What you can do | Where it is set | | - | - | - | | Inbound | Read `P-Asserted-Identity` and `Diversion` into prompt variables | Agent, `sip_header_variables` | | Outbound | Send your own headers on every call the agent places | Agent, `sip_headers` | | Transfer | Send your own headers on the call to the transfer target | Transfer tool, `sip_headers` | ## Inbound header variables Bolna passes two headers from your carrier's INVITE to the agent: `P-Asserted-Identity` and `Diversion`. Other headers on the INVITE do not reach the agent. To use either one, map it to a variable name on the agent; the variable then appears in `recipient_data` and in the prompt as `{variable_name}`. The variable holds the header's value exactly as your carrier sent it, display name and parameters included, for example `"Asha" `. If the INVITE carries the same header more than once, the first one is used. A header that is not on a call leaves its variable empty; the call still goes ahead. Open the agent, go to the **Calling** tab and set **Telephony Provider** to **SIP Trunk**. The **SIP Header Variables** section appears under **Inbound Calling**. Click **Add header**. Enter the header name on the left, exactly `P-Asserted-Identity` or `Diversion`, and the variable name on the right, for example `caller_identity`. Header names are case-sensitive. Any other header name shows a warning, because a SIP trunk never passes it to the agent. Agent Calling tab, SIP Header Variables section under Inbound Calling, with P-Asserted-Identity mapped to caller_identity and Diversion mapped to diverted_from Reference it in the prompt or welcome message, for example `The network reports the caller as {caller_identity}.` The **Ingest lookup key** below the mappings can pick one of these headers to look the caller up in your [caller data](/docs/customizations/identify-incoming-callers) instead of `from_number`. The lookup uses the header's full value, so your data source has to store it in the same form. ## Outbound header templates Headers you set on the agent go on every INVITE the agent places through a trunk: single calls, batches and workflow calls. Each value is a template, and `%(name)s` in it is replaced with that call's variable, typically from the `user_data` you pass when [placing the call](/docs/sip-trunking/outbound-calls). ```json theme={"system"} { "X-Account-Id": "%(account_id)s", "X-Campaign": "renewals-q4" } ``` A call with `"user_data": {"account_id": "48219"}` sends `X-Account-Id: 48219` and `X-Campaign: renewals-q4`. A header whose value comes out empty, because the call has no such variable, is left off that call. Save the agent with **Telephony Provider** set to **SIP Trunk**, then open its **Tools** tab. **SIP headers on outbound calls** is at the bottom of the **Function Tools for LLM Models** card. Click **Add header**, enter the name and the value, and click **Save headers**. Agent Tools tab, SIP headers on outbound calls section, with an X-Account-Id header whose value is %(account_id)s, and Add header and Save headers buttons A call uses the headers saved on the agent version it runs on. ### Header rules These apply to outbound and transfer headers on SIP trunk agents. Bolna refuses to save a header whose name or template breaks them. A value that breaks them only once a variable is filled in, for example by running past 120 characters, is left off that call. | Rule | Detail | | - | - | | Name | Letters, digits and dashes, up to 64 characters. Each name once, ignoring case. | | Value | Up to 120 characters after substitution. No quotes, pipes, curly braces, backslashes or control characters. | | Reserved for Bolna | Any name starting with `X-Bolna-`, plus `X-Client-Call-Id` and `X-Trunk` | | Owned by SIP | `Via`, `From`, `To`, `Call-ID`, `CSeq`, `Contact`, `Route`, `Record-Route`, `Max-Forwards`, `Content-Length`, `Content-Type`, `Authorization`, `Proxy-Authorization`, `P-Asserted-Identity`, `P-Preferred-Identity`, `Remote-Party-ID`, `Privacy`, `Diversion`, `History-Info`, `Refer-To`, `Referred-By`, `Replaces`, `Require`, `Proxy-Require`, `Supported`, `Allow`, `Session-Expires`, `Min-SE`, `Expires`, `Event` | ### Headers Bolna adds Every outbound and transfer INVITE on a trunk also carries two headers of Bolna's own, which you can use to match your carrier's records to Bolna's: | Header | Value | | - | - | | `X-CLIENT-CALL-ID` | The Bolna execution ID of the call | | `X-TRUNK` | The trunk ID | ## Transfer-leg headers When an agent on a SIP trunk [transfers a call](/docs/sip-trunking/call-transfer), the call to the transfer target carries the headers set on that transfer tool, not the agent's outbound headers. Values use the same `%(name)s` templates and the same rules. On an inbound call, the variables include your mapped header variables and any caller data, so you can forward what the carrier told you about the caller. In the dashboard, the **Transfer Call** tool shows **Custom SIP headers** when the destination is **SIP trunk (URI)**. Through the API, a transfer tool's `sip_headers` apply whichever destination type it uses. ## Using the API Set both agent-level fields with the [agent PATCH](/docs/api-reference/agent/v2/patch_update) endpoint. Send `null` to clear either one. ```bash Inbound header variables theme={"system"} curl -X PATCH https://api.bolna.ai/v2/agent/ \ -H "Authorization: Bearer " \ -H "Content-Type: application/json" \ -d '{ "agent_config": { "sip_header_variables": { "P-Asserted-Identity": "caller_identity", "Diversion": "diverted_from" } } }' ``` ```bash Outbound header templates theme={"system"} curl -X PATCH https://api.bolna.ai/v2/agent/ \ -H "Authorization: Bearer " \ -H "Content-Type: application/json" \ -d '{ "agent_config": { "sip_headers": { "X-Account-Id": "%(account_id)s", "X-Campaign": "renewals-q4" } } }' ``` A header that breaks a rule is refused with `400`, for example `reserved header X-Trunk`. A transfer tool carries its headers in its `param`, next to the destination: ```json theme={"system"} { "call_transfer_number": "+918045670000", "sip_headers": { "X-Caller-Identity": "%(caller_identity)s" } } ``` See [Transfer calls](/docs/tool-calling/transfer-calls) for the rest of the transfer tool. This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.