GetExternalContactAction failing to parse phone number parameter

Architect & Flows
1

Need some help troubleshooting the GetExternalContactAction in Architect. i’m trying to look up a customer by phone number but the action keeps failing with a 400 Bad Request. the input parameter is set to system.caller.phoneNumber, but the external API seems to be receiving undefined or an empty string. here’s the JSON config i’m using for the data action:

{
 "inputs": {
 "phone": "${system.caller.phoneNumber}"
 },
 "settings": {
 "url": "https://api.example.com/lookup"
 }
}

any idea why the expression isn’t resolving? thanks.