I just tried to create a call without setting up a phone number as you suggested. It seems to be blocked by the same validation layer when I don't add the "1" in front of 800.

{
  "message": [
    "phoneNumber.twilioPhoneNumber must be a valid phone number in the E.164 format. Hot tip, you may be missing the country code (Eg. US: +1)."
  ],
  "error": "Bad Request",
  "statusCode": 400
}