> For a complete page index, fetch https://docs.synthflow.ai/llms.txt. For full documentation content, fetch https://docs.synthflow.ai/llms-full.txt.

## Use SIP X-Headers in prompts, actions, and transfers

Synthflow now accepts arbitrary `X-*` SIP headers on inbound calls. Any custom header your SBC, PBX, or carrier attaches to the SIP INVITE is automatically extracted and made available as a variable throughout the call lifecycle — in prompts, custom actions, and outbound transfer headers.

See the full guide: [Use SIP X-Headers in Prompts, Actions & Transfers](/docs/calls/sip-x-headers-in-prompts).