phreaking
Phreaking is an older method to avoid telephone costs, which is causally related to the switching techniques in telephone networks and there in particular to signaling. The creativity of phreakers, i.e. those who have mastered phreaking, has not been limited to replicating signaling sequences.
Phreakers analyze and modify the vulnerabilities of various dial-up techniques and forms of payment, including phone cards or calling cards, and consistently use the findings to make their own free calls. As for signaling, telecommunication networks in the U.S., Canada, Australia, and China use the older C5signaling, which is inner-band signaling with two-tone and multi-tone procedures in the voice channel, so it does not use an independent signaling channel, such as the No. 7 signaling system( CCS7).
The control signals and the control sequences of the C5 signaling can thus be filtered out directly from the voice channel and misused for their own purposes. Since the operating companies change the control sequences because of the phreaking, the phreakers work with frequency scanners that automatically scan the frequencies, record the sequences and put the whole thing into a dialer. Phreakers can be detected by means of carrier monitoring.
Phreaking, which is particularly interesting for international connections, is a global problem. Nationally, phreaking is difficult due to the out-of-band signaling used.