Custom Local Area Signaling Services (CLASS) are PSTN telephony intelligent network services beyond simple voice transfer, such as caller ID (automatic number identification (ANI)), caller number blocking, automated call return, call blocking or screening, TCAP services, etc. Various IXCs offer other services. The public switched telephone network (PSTN) is the concatenation of the worlds public circuit-switched telephone networks, in much the same way that the Internet is the concatenation of the worlds public IP-based packet-switched networks. ... A telephone handset A touch-tone telephone dial Telephone The telephone or phone (Greek: tele = far away and phone = voice) is a telecommunications device that transmits speech by means of electric signals. ... The Intelligent Network or I.N., as it is more commonly referred to, is a network architecture for both fixed and mobile telecommunication networks. ... In the United States, Interexchange carrier (or IXC) is a legal and regulatory term for a telecommunications company, commonly called a long-distance telephone company, such as AT&T, MCI, and Sprint. ...

The services rely on the ITU-T standard Signalling System 7 (SS7, European equivalent is C7) connectvity between network switches (SSPs) and databases (800 Database, LNPDB, LIDB, etc.) to operate. The CLASS standards were defined by Bellcore in the 1970s. The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland. ... Signalling System #7 (SS7) is a set of telephony protocols which are used to set up the vast majority of worlds telephone calls. ... Telcordia Technologies, formerly Bellcore, is an American telecommunications company created in 1984 after the breakup of AT&T. It was split from the original Bell Labs as part of the negotiated consent decree with the US government, and served Research & Development and standards setting functions for the resulting seven Baby...

Other services such as call forwarding, three-way calling, call waiting, or speed dialing are part of custom calling rather than CLASS.

See also: enterprise telephony, VoIP. IP Telephony, also called Internet telephony, is the technology that makes it possible to have a telephone conversation over the Internet or a dedicated Internet Protocol (IP) network instead of dedicated voice transmission lines. ...