Architect GetExternalContactAction returning empty payload on phone lookup

Problem
We’re wiring up a quick customer lookup before the queue dashboard loads. The flow drops into GetExternalContactAction with the phone number variable, but the external service just bounces back a 404.

{
 "inputs": {
 "LookupField": "PhoneNumber",
 "LookupValue": "{{flow.contact.phone_number}}"
 }
}

The response object stays completely blank. It’s definitely a schema mismatch.