POST /api/v2/conversations/calls 400 on valid phone format

Trying to kick off an outbound call via the REST API but getting a 400 Bad Request. The error says the participant address is malformed even though the number is just a standard E.164 string. Here is the payload I’m sending:

{
 "from": { "address": "+15551234567", "phoneNumberType": "WORK" },
 "to": [{ "address": "+12125550199", "phoneNumberType": "MOBILE" }]
}

The docs don’t list any other required fields for a basic call. What am I missing?