Cobra has used two different types of call signal: analog and digital. A digital call signal is used on models with VibrAlert, and is necessary in order to activate the vibrate feature. Models without VibrAlert use an analog call signal.
Digital call signals are not fully compatible with analog call signals. If your Cobra radio has VibraAlert, then it has a digital call signal that will only work properly with other Cobra VibrAlert models.