POST /api/v2/conversations/calls returns 400 with malformed participant address

Trying to initiate a call via the REST API. The docs say the participant address needs to be a valid phone number. I’m sending +15551234567 in the JSON payload, but the server keeps rejecting it with a 400 error. The response body says the address is malformed.

Here is the :

{
 "to": [{"address": "+15551234567", "name": "Test"}],
 "from": {"address": "+15559876543"}
}

I’ve checked the token. It’s valid. What am I missing here?