POST /api/v2/conversations/calls returns 401 for agent

Trying to kick off an outbound call via the API on behalf of a specific agent. The endpoint is POST /api/v2/conversations/calls. I’m passing the agent’s ID in the fromId field and using an OAuth token generated via client credentials. The response is a hard 401 Unauthorized. I’ve checked the token scope and it looks fine. Here’s the payload:

{
 "fromId": "agent-id-123",
 "toAddress": "+447700900000",
 "mediaType": "voice"
}

Is there a specific scope I’m missing?