Technical characteristics of tones for the telephone service are listed in ITU-T Recommendation Q.35 (1988). Infortunately that document is not freely available (can be ordered from ITU-T if you are willing to pay), so the information available in the following documents might be useful if you are looking for free information on telephone line signaling.
The following telephone circuits are mainly ment to give ideas for your own desigs. Many of the circuits are quite propably do not meet your local telephone network terminal equipment specs (they vary from country to country) and are quite propably poorly designed (this applies as well to many circuits in books and hobby magazines). 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.
For information what is allowed to be connecte to telephone line the reader form USA should check FCC part 68 regulations and European reader should check NET4 (ETS 300 001) regulations. Good information on telephone equipment regulations can be found from Compliance Engineering Magazine website.
Telephone bugging is an illegal practice. The following circuits are for educational use. They are illagal in many countries.
