GetExternalContactAction returning 404 on valid phone number lookup

We are trying to use the GetExternalContactAction in our Architect flow to pull customer data based on the caller’s phone number. The endpoint is configured to hit our internal API, but we keep hitting a 404 Not Found error in the logs. The phone number is passed as a string, but I’m wondering if the format matters. Here is the JSON config we are using for the action. Any ideas why it might be failing?