POST /api/v2/conversations/calls returns 400 Bad Request on outbound call

Stuck on the outbound call initiation endpoint. We’re trying to kick off a call from our middleware when a specific workflow triggers, but the API keeps rejecting the payload.

Here’s the call:

POST https://api.mypurecloud.com/api/v2/conversations/calls

Payload:

{
 "to": "+61298765432",
 "from": "+61212345678",
 "callFlowId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
 "wrapUpCode": "default"
}

The response is a 400 Bad Request. The error object says "message": "Invalid request body" but doesn’t specify which field is wrong. I’ve verified the call flow ID exists and the phone numbers are E.164 formatted. The user token has conversation:call:write permissions.

Is there a required field missing here? The docs show to, from, and callFlowId as the main ones. We’ve tried adding attributes but that just makes it fail harder.