POST /api/v2/conversations/calls returns 400 Bad Request - invalid 'to' address

Hey folks,

I’m trying to kick off an outbound call programmatically on behalf of an agent using the Conversations API. The goal is to have the system dial out and then bridge it to the agent once the customer answers.

Here’s the payload I’m sending to POST /api/v2/conversations/calls:

{
 "from": {
 "phoneNumber": "+18005550199",
 "name": "Customer Service"
 },
 "to": [
 {
 "phoneNumber": "+12125550148",
 "name": "John Doe"
 }
 ],
 "provider": {
 "id": "my-telephony-provider-id"
 },
 "callbackNumber": "+18005550199"
}

I keep getting a 400 Bad Request back. The error message is pretty vague:

{
 "message": "Validation Failed",
 "details": [
 {
 "message": "to[0].phoneNumber is invalid"
 }
 ]
}

The phone number is definitely valid E.164. I’ve tested it in the UI and it works fine. I’ve also checked the from number and it’s whitelisted in the provider.

Am I missing a required field? Or is there a specific format the API expects for the to object that’s different from the UI? I’ve been staring at the docs for an hour and I’m going in circles.

Any pointers would be appreciated.