"errorCode" : "CMN-101" on CallOut from C# SDK

Running this code from my C# program...

callOutRequest.from = new MakeCallOutCallerInfoRequestFrom {deviceId = fromDeviceId};
callOutRequest.to = new MakeCallOutCallerInfoRequestTo {phoneNumber = toNum};
CallSession session = await rc.Restapi().Account().Telephony().CallOut().Post(callOutRequest);

Resulting in this...

Content: {
   "errors" : [
      {
         "errorCode" : "CMN-101",
         "message" : "Parameter [phoneNumber] value is invalid"
      }
   ]
}

The "Phone number is a valid, 10 digit phone number". The "device Id" is a 12 digit number taken from my RC account.

What's wrong with what I'm doing here?

Thanks,

Shelby