The Batphone
A device which is used by Batman and Police Commissioner Gordon to communicate.
The Batphone was located in three places; Bruce's study in Wayne Manor, a desk in the Batcave and a mobile phone in the Batmobile. There was only one in the Commissioner's office. He kept it on a special desk under a large glass cover.
The batphone was a direct line to Commissioner Gordon's office. The phone was red with a button in the center where the dials would have been. To make a call, either Batman or the Commissioner would press the button. When a call was received, it would beep until answered.
Other people would use the Batphone as well; Robin or Alfred (if Batman wasn't around) and Chief O'Hara would answer (reluctantly) if the Commissioner wasn't there. Batgirl used the phone in the Commissioners office on occasion to speak to Batman.