​ To learn more, see the sources I considered: [[1] Set & parse custom SIP headers](https://docs.retellai.com/build/telephony/sip-headers) ​

### Answer You can use dynamic variables for custom SIP header values in call transfers. [(1) ]() For call transfers, custom SIP headers must start with

X-

. You can use dynamic variable syntax for your custom SIP header value, so you can pass information extracted from the call to the receiving party. For example, in your transfer node configuration, you would set a custom SIP header like: - **Header name**:

X-First-Name

- **Header value**:

{{first_name}}

For detailed configuration: - If you're using Conversation Flow agents, see the [call transfer node]() documentation. - If you're using Single/Multi-prompt agents, see the [call transfer function]() documentation. **Important limitation**: For cold transfers with transferee number, different telephony providers may or may not honor custom SIP headers in the REFER request. Twilio, for example, does not honor custom SIP headers in the REFER request, so SIP headers set in the transfer call tool will not work for cold transfer with transferee number when using Twilio.

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer If the dynamic variable

{{first_name}}

is showing literally instead of being substituted, there are a few things to check: 1. **Ensure the variable is properly defined and populated**: Dynamic variables must be set either via

retell_llm_dynamic_variables

when creating the call, through the inbound call webhook, or collected during the conversation. [(1) ]() 2. **Check variable naming**: The spaces around the variable name will be trimmed when evaluating the variable, but make sure the variable name matches exactly (case-sensitive) what was defined. 3. **Verify the variable exists at transfer time**: If a dynamic variable has no assigned value, it remains in its raw form with the curly braces intact. To check if a variable is set in conversation flow conditions, you can use:

Equation: {{first_name}} exists
Result: True if variable is set

If the variable isn't being set properly before the transfer occurs, you'll see the literal

{{first_name}}

text. You may want to confirm that

first_name

is either: - Passed in via

retell_llm_dynamic_variables

at call creation - Set via inbound call webhook - Collected and stored as a dynamic variable during the conversation flow Would you like help troubleshooting how the variable is being set in your specific flow?

​ To learn more, see the sources I considered: [[1] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.