code: func (c *Call) EndedUnsuccessfully() bool {
  	// https://www.twilio.com/docs/api/rest/call#call-status-values
  	switch c.Status {
  	case StatusCanceled, StatusFailed, StatusBusy, StatusNoAnswer:
  		return true
  	default:
  		return false
  	}
  }
docs: // EndedUnsuccessfully returns true if the Call has reached a terminal state
  // and that state isn't "completed".