The origins of phone phreaking trace back at least to AT&T's implementation of fully automatic switches. These switches used tone dialing, a form of in-band signaling, and included some tones which were for internal telephone company use. One internal-use tone is a tone of 2600 Hz which causes a telephone switch to think the call had ended, leaving an open carrier line, which can be exploited to provide free long-distance, and international calls. At that time, long-distance calls were more expensive.