Doorbell

A doorbell is a device which is typically installed to or near a door. When a visitor presses a button, the bell rings inside the building, alerting the presence of the visitor. 
In modern technology, a button is pressed completing a circuit which in response makes a sound. 
History.
Older doorbells were fully mechanical. Newer electrical doorbells were installed by wires and use a push-button to activate the sound. The latest doorbells have wireless connections although older ones may be wired.