.NET SDK POST /api/v2/conversations/calls 400 Bad Request on participant address

The docs say the from address should be a string. I’m passing exactly that in the C# model, but the API keeps rejecting it with a 400. The error message is vague about the “malformed participant address”.

Here’s the payload I’m sending:

{
 "from": {
 "address": "sip:alice@mydomain.com",
 "name": "Alice"
 },
 "to": [
 {
 "address": "sip:bob@mydomain.com",
 "name": "Bob"
 }
 ]
}

The .NET SDK serializes this fine. I’ve checked the token scopes and they look correct. Is there a specific format for the address field that isn’t in the docs? I’ve tried removing the sip: prefix but that just breaks the call setup entirely. The endpoint is definitely /api/v2/conversations/calls.