Connecting non-approved equipment to public telephone network is illegal in many countries. If you connect non-approved circuit which do not meet the specs to the public telephone network the telephone do not work properly with them and they also cause safety hazards. Good information on telephone equipment regulations can be found from Compliance Engineering Magazine website. The following circuits are for educational use. They are illagal in many countries.
Telephone hacking The following telephone hacking circuits are here only for educational purposes.
Most of them don't work anymore and connecting them to telephone network is illegal.