POST /api/v2/conversations/calls 400: malformed participant address with valid tel URI

Hit a wall with the outbound call API. Trying to kick off a call via POST /api/v2/conversations/calls. The request returns a 400 Bad Request with the message “Malformed participant address”.

Here’s the payload:

{
 "participants": [
 {
 "name": "Test Call",
 "id": "agent-uuid-here",
 "to": { "phoneNumber": "+2348012345678" }
 }
 ]
}

The agent UUID is valid. The phone number is E.164. I’ve tried wrapping it in tel: prefix, changing the key to extension, and even using the full address object structure. Nothing sticks. The docs are vague on what exactly constitutes a malformed address in this context.

Is there a specific format required for the to object that isn’t documented? Or is this a known issue with certain regions? The error doesn’t specify which part is malformed. Just want to get this moving.