Personal Connection API 400: Invalid recipient format for outbound call

Trying to trigger an outbound call via the CXone Personal Connection API. The goal is to inject a traceparent header for OTel span propagation, but the basic call is failing.

Endpoint: POST /api/v2/outbound/campaigns/{campaignId}/executions

Payload:

{
 "campaignId": "abc-123",
 "contact": {
 "phoneNumber": "+15550199",
 "traceContext": "00-..."
 }
}

Returns 400 Bad Request. Error says invalid recipient. The number is valid E.164. What’s wrong with the structure?