POST /api/v2/conversations/calls 400 Bad Request

Trying to initiate a call via the API but keep hitting a 400 error. The docs say the participant address is malformed, but the format looks right to me. Here’s the payload I’m sending:

{
 "to": { "phoneNumber": "+15550199" },
 "from": { "phoneNumber": "+15550100" },
 "type": "call"
}

The numbers are valid. No extra spaces. What am I missing?