400 Bad Request on POST /api/v2/conversations/calls - malformed participant address

Is it possible to dial an external number using a plain string in the participant object? i’m running a PowerShell script to trigger outbound calls via Invoke-RestMethod. The token works fine for other GET calls. sending this JSON payload:

{
 "to": [{"phoneNumber": "+61298765432", "name": "Test"}],
 "from": {"phoneNumber": "+61212345678"}
}

getting a 400 error back saying the participant address is malformed. checked the docs and it looks right. am i missing a schema version or something?