Some telephones work with certain hearing aids to make sounds louder and remove background noise. And some auditoriums, movie theaters, and other public places are equipped with special sound systems that send sounds directly to your ears.
Alerts such as doorbells, smoke detectors, and alarm clocks can give you a signal that you can see or a vibration that you can feel. For example, a flashing light can let you know someone is at the door or on the phone.