Bell automated this process by sending "in-band" signals. Since the one thing the long-distance trunks were definitely able to do was send voice-frequency signals, the Bell System used a selection of tones sent over the trunks to control the system. When calling long-distance, the local end-office switch would first route the call to a special switch which would then convert further dialing into tones and send them over an appropriately selected trunk line (selected with the area code). A similar machine at the far end of the trunk would decode the tones back into electrical signals, and the call would complete as normal.