The ring that could not be ignored
A ringing telephone was designed to demand attention. For many decades a subscriber had only crude ways to avoid an unwanted call: not answering, taking the receiver off the hook, or asking the telephone company for a different number. The wish to refuse certain callers is old, but the tools for doing it came late, because they depended on the exchange knowing who was calling.
Early privacy measures
The first protection was simply keeping a number out of the directory. Telephone companies offered unlisted or unpublished numbers, sometimes for an extra fee, for people who wanted fewer calls. This blocked casual callers but did nothing against someone who knew the number.
Party lines, in which several households shared one circuit, created the opposite problem. Neighbors could hear one another’s calls, and individual rings were distinguished by different patterns, such as long and short rings. Privacy and control were limited by design.
Operators acted as informal filters in manual exchanges. They knew which subscribers had asked not to be disturbed at certain hours, and could take a message. That informal gatekeeping disappeared with automatic exchanges.
Harassing calls and the first network responses
As automatic dialing spread, anonymous nuisance calls became a recurring complaint. Telephone companies responded in several ways. They offered to change a number, to put a notice on the line, or to trace calls in serious cases. Tracing in the era of mechanical switches was slow and laborious, because technicians sometimes had to follow the path of a call through each switching stage by hand while it was in progress. Electronic switching made it easier to record where a call came from.
Selective call rejection
Computer-controlled exchanges made new services practical. In the 1980s, North American telephone companies developed a suite of custom calling features, often grouped under the name CLASS (Custom Local Area Signaling Services). One of them, usually called selective call rejection, allowed a subscriber to build a list of numbers whose calls would be refused, typically by entering a code and then the numbers. Callers on the list heard a message saying the subscriber was not accepting calls.
Other features dealt with the same problem from different angles. Call trace let a subscriber report the last call to the telephone company, which could pass the details to authorities. Anonymous call rejection blocked calls whose number was withheld. Caller identification, a service that displayed the incoming number, let people choose not to answer, and also provided the information needed for a blocking list. The services varied by country and carrier, and not every subscriber could buy every feature.
Telemarketing and do-not-call rules
From the 1980s, falling long-distance prices and automatic dialing made telemarketing and, later, recorded-message calls much cheaper. Complaints grew. Many governments responded with legal rules. In the United States, a 1991 law regulated automated and unsolicited calls, and a national do-not-call registry, established by the Federal Trade Commission, started in 2003, letting households list their numbers to reduce sales calls. Other countries introduced similar registries or rules. Such rules do not stop all unwanted calls, particularly from callers who ignore the law, and the details differ by jurisdiction.
Mobile phones and apps
Mobile phones moved blocking into the handset, and carriers on cellular networks added filtering of their own. Phone makers added a list of blocked numbers in the settings, and calls from numbers on the list are rejected or sent directly to voicemail. On smartphones, blocking became a standard feature in the 2010s. Apps and carrier services began to offer lists of suspected nuisance numbers, using reports from users, and some use automatic analysis to label a call as likely spam. These systems are not perfect, since they can wrongly flag legitimate callers and cannot always catch new numbers.
Spoofing and authentication
A problem for number-based blocking is spoofing: a caller can display a false number. When unwanted callers can change the number easily, blocking a single number is of limited use. In response, telecommunications regulators and carriers in several countries have worked on technical standards to authenticate that a caller’s number is genuine. One set of standards, known as STIR/SHAKEN, began to be deployed by United States carriers around 2020 and 2021. Results have been mixed, and unwanted calls remain a concern.
The logic resembles that of network firewalls in computing: decide, using a list or a pattern, which traffic to admit, a subject that has grown into cybersecurity.
What the history shows
Call blocking is a case where the original design of the network did not anticipate how it would be abused. The telephone was built on the idea that a call should be easy to place, since the aim was to connect people. Over time, the network had to add ways for the receiver to decline. Each step, from an unlisted number to selective rejection to a smartphone list, gave the person being called a little more control, and each step was followed by new methods of reaching them.