Using variables in custom functions

You are agent Sam speaking to {{customer_name}}.
The agent ID is "{{agent_id}}".
The call ID is "{call_sid}".
The customer's phone is "{{to_number}}".

{
  "name": "log_interaction",
  "description": "Log customer interaction to CRM",
  "parameters": {
    "type": "object",
    "properties": {
      "to_number": {
        "type": "string",
        "description": "Customer's phone number"
      },
      "agent_id": {
        "type": "string",
        "description": "Agent ID"
      },
      "customer_name": {
        "type": "string",
        "description": "Customer's name"
      },
      "customer_address": {
        "type": "string",
        "description": "Customer's address"
      }
    },
    "required": ["to_number", "agent_id", "customer_name", "customer_address"]
  },
  "key": "custom_task",
  "value": {
    "method": "POST",
    "param": {
      "to_number": "%(to_number)s",
      "agent_id": "%(agent_id)s",
      "customer_name": "%(customer_name)s",
      "customer_address": "%(customer_address)s"
    },
    "url": "https://my-api-dev.xyz",
    "api_token": "Bearer your_token",
    "headers": {}
  }
}

{
  "to_number": "+19876543210",
  "agent_id": "7d86b904-da64-4b8e-8f51-6fef2c630380",
  "customer_name": "Bruce",
  "customer_address": "221B Baker Street"
}