Dial tone

Modern electronic telephones may store the digits as they are entered, and only switch off-hook to complete the dialing when the subscriber presses a button.

[1] The Bell Telephone Manufacturing Company (BTMC) in Antwerp, Belgium, Western Electric's international subsidiary, first introduced dial tone as a standard facility with the cutover of the 7A Rotary Automatic Machine Switching System at Darlington, England, on 10 October 1914.

[2] Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical devices such as motor-generators or vibrators.

Cellular telephone services do not generate dial tones as no connection is made until the entire number has been specified and transmitted.

Private branch exchanges (PBXs) or key telephone systems also play a dial tone to station users.

It may be the same type as used by the public switched telephone network (PSTN), or it may be a different tone to remind users to dial a prefix or select by another method an outside line.

It is maintained only so that an attached phone can dial the emergency telephone number (such as 911, 112 or 999), in compliance with the law in most places.