### Answer For inbound phone calls, the system default dynamic variable for the caller's phone number is

{{user_number}}

. [(1) ]() For phone calls specifically, these system default dynamic variables are available: - **{{direction}}**: inbound or outbound - **{{user_number}}**: user phone number (for inbound calls this is the

from_number

, for outbound it's the

to_number

) - **{{agent_number}}**: agent phone number (for inbound calls this is the

to_number

, for outbound it's the

from_number

) So in your custom function, you would use

{{user_number}}

as the dynamic variable to pass the inbound caller's phone number as a query parameter to your webhook. For custom functions, you can make GET, POST, PUT, PATCH, and DELETE requests and add custom headers and query parameters.

​ To learn more, see the sources I considered: [[1] Platform Changelogs | Retell AI](https://www.retellai.com/changelog) ​

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

Ok I'm trying to make it an mcp tool. I added the tool to the mcp for the single prompt agent. But when she tried to call it, she put a random phone number, not the {{user_number}} @AI Evy

### Answer When using MCP tools, the LLM resolves parameter values based on descriptions you provide—it doesn't automatically inject system dynamic variables like

{{user_number}}

unless you explicitly configure them as const values rather than letting the LLM decide. [(1) ]() To ensure

{{user_number}}

is passed correctly, you have two options: Option 1: Use a const value in query parameters When setting up your MCP tool, add a query parameter and toggle it to use a const value instead of a description. Set the const value to

{{user_number}}

. This will be applied directly without LLM interpretation. [(2) ]() Option 2: Switch to a Custom Function Custom functions give you more explicit control. When defining query parameters, you can switch between "parameter description" (resolved by LLM) or "const value" (applied directly) . For const values, you can use dynamic variables like

{{user_number}}

which will be evaluated at runtime. The key difference: when you provide a description, the LLM decides what value to pass based on conversation context (which is why it may hallucinate a phone number) . When you use a const value with

{{user_number}}

, it bypasses LLM interpretation and uses the actual system variable. [(3) ]() [(2) ]() Also, make sure your prompt explicitly instructs the agent to use the caller's phone number from the system, not to invent one. [(1) ]()

​ To learn more, see the sources I considered: [[1] MCP](https://docs.retellai.com/build/single-multi-prompt/mcp) [[2] Integrate any system with custom function](https://docs.retellai.com/build/single-multi-prompt/custom-function) [[3] 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.

### Answer Your setup looks correct for using

{{user_number}}

as a const value in query parameters. However, there's an important consideration: if you're testing via web call or the chat interface, the

{{user_number}}

variable will be empty because there's no actual phone number associated with the session. [(1) ]() The phone call system variables like

{{user_number}}

,

{{agent_number}}

, and

{{direction}}

are only available for phone calls—not web calls or chat sessions. To verify this is the issue: - Test with an actual phone call (inbound or outbound) rather than web testing [(2) ]() - The

{{user_number}}

variable represents the user's phone number: for inbound calls it's the

from_number

, for outbound it's the

to_number

[(1) ]() If you need to test without a real phone call, you have two options: 1. **Use agent-level default dynamic variables**: Set a default value for a custom variable in your agent configuration that will be used as a fallback when the variable isn't provided in the call request 2. **Use the Extract Dynamic Variables function**: If you need the agent to capture a phone number the user speaks during the conversation (rather than the caller ID) , you can use the extract dynamic variables feature to save it and then reference it in your custom function [(2) ]() The agent saying "phone_number field wasn't provided" suggests the

{{user_number}}

variable evaluated to empty/null during your test, which confirms this is likely a web call testing issue.

​ To learn more, see the sources I considered: [[1] Dynamic Variables](https://docs.retellai.com/build/dynamic-variables) [[2] Custom Function Not substituting variable](https://discord.com/channels/1186424376300679209/1450867178906714123) ​

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