POST /api/v2/conversations/calls returning 400 — malformed participant address

I’m trying to programmatically place an outbound call using the REST API. The request looks correct but I get a 400 Bad Request with the message malformed participant address. Here is the payload I’m sending:

{
 "participants": [{
 "name": "Test Agent",
 "address": "user:12345",
 "type": "agent"
 }]
}

The user ID is valid and the user is online. What is the correct format for the address field?