Nuisance calls encompass any type of unwanted, unsolicited, telephone call. Common types of nuisance calls include prank calls, telemarketing calls, and silent calls. Obscene phone calls and other threatening calls are criminal acts in most jurisdictions, particularly when hate crime is involved.
Caller ID provides some protection against unwanted calls, but can still be turned off by the calling party. Even where end-user Caller ID is not available, calls are still logged, both in billing records at the originating telco and via automatic number identification, so the perpetrator's phone number can still be discovered in many cases. However, these do not provide complete protection: harassers can use payphones, in some cases, automatic number identification itself can be spoofed or blocked, and mobile telephone abusers can (at some cost) use "throwaway" mobile phones or SIMs. Even in these cases, with sufficient law enforcement and telco effort, persistent abusers can often be tracked down by technical means. However recently wiretapping by law enforcement in many jurisdictions; especially in the United States has helped circumvent such disguise-like methods by the harasser.
Call screening is another countermeasure against unsolicited calls.
Most telephone companies have a department devoted to handling complaints regarding unsolicited phone calls; in criminal cases such as obscene or threatening phone calls, many police departments have dedicated resources devoted to dealing with such cases.
See also 
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