A telephone number is a sequence of digits assigned to a fixed-line telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices for data transmission via the public switched telephone network (PSTN) or other private networks. A telephone number serves as an address for switching telephone calls using a system of destination routing. Telephone numbers are entered or dialed by a calling party on the originating telephone set which transmits the sequence of digits in the process of signaling to a telephone exchange. The exchange completes the call either to another locally connected subscriber or via the PSTN to the called party. The concept of using telephone numbers instead of subscriber names when connecting calls was developed and first used between 1879 and 1880 in Lowell, Massachusetts, United States, for the purpose of ease of training new telephone operators.