Cramming is the addition of charges to a subscriber's telephone bill for services which were neither ordered nor desired by the client, or for fees for calls or services that were not properly disclosed to the consumer. These charges are often assessed by dishonest third-party suppliers of data and communication service that phone companies are required, by law, to allow the third-party to place on the bill.
Slamming is any fraudulent, unauthorized change to the default long-distance/Local carrier or DSL internet service selection for a subscriber's line, most often made by dishonest vendors desirous to steal business from competing service providers.
False Answer Supervision is a misconfiguration of telco equipment, by negligence or design, which causes billing to start as soon as the distant telephone begins ringing, even if a call is busy or no answer. The cost is typically subtle but recurring as subscribers repeatedly pay some small amount for calls which were never completed.
Autodialers may be used for a number of dishonest purposes, including telemarketing fraud or even as War dialing. War dialers take their name from a scene in the 1983 movie WarGames in which a 'cracker' programs a home computer to dial every number in an exchange, searching for lines with auto-answer data modems. Sequential dialing is easy to detect, pseudo-random dialing is not.
Autodiallers are also used to make many short-duration calls, mainly to mobile devices, leaving a missed call number which is either premium rate or contains advertising messages, in the hope that the victim will call back. This is known as Wangiri (literally, "One (ring) and cut") from Japan where it originated.
Caller ID spoofing can be used to fraudulently impersonate a trusted vendor (such as a bank or credit union), a law enforcement agency or another subscriber. These calls may be used for vishing, where a scammer impersonates a trusted counterparty in order to fraudulently obtain financial or personal information.
Call clearing delays in some United Kingdom exchanges may be abused to defraud. For obscure historical reasons, the system was designed so that a called party could hang up a call and immediately pick it up from another extension without it being disconnected. A fraudster would call a household and impersonate a bank or police; when the householder hung up and then attempted to call police or contact the impersonated party, the fraudster would still be on the line because the original call never properly ended
Everyday hundreds of calls are received on US mainland, offering the recipients grant money from the Federal Govt, in lieu of a "small administration fee" though there are no fees associated with applying or receiving a Government Grant