Genesys Cloud POST /api/v2/conversations/calls 400 Bad Request - participant address

Trying to spin up a call via the API but hitting a 400. The docs say the participant needs a to address, and I’ve got that in the payload. Still getting “malformed participant address” back. Here’s the JSON I’m sending:

{
 "participants": [
 {
 "from": { "phoneNumber": "+15550000000" },
 "to": { "phoneNumber": "+15559999999" }
 }
 ]
}

What am I missing here?