Architect GetExternalContactAction returning empty result

I’m trying to look up a customer by phone number using the GetExternalContactAction in Architect. The integration is set up and working in Postman, but the flow returns empty fields. Here’s the config I’m using.

{
 "key": "${contact.phone.number}",
 "type": "phone"
}

The variable definitely has a value. I’m just getting null back in the subsequent set contact attributes step.