mathematics

Reverse telephone directory

Reverse telephone directory is a mathematics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Reverse telephone directory rather than just read about it. In short: A reverse telephone directory (also known as a gray pages directory, criss-cross directory or reverse phone lookup) is a collection of telephone numbers and associated customer details. However, unlike a standard telephone directory, where the user uses customer's details (such as name and address) in order to retrieve the telephone number of that person or business, a reverse telephone directory allows users to sea…

Key takeaways

Reverse telephone directory belongs to mathematics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Reverse telephone directory to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Reverse telephone directory from memory before moving on to harder problems.

Reference excerpt

A reverse telephone directory (also known as a gray pages directory, criss-cross directory or reverse phone lookup) is a collection of telephone numbers and associated customer details. However, unlike a standard telephone directory, where the user uses customer's details (such as name and address) in order to retrieve the telephone number of that person or business, a reverse telephone directory allows users to search by a telephone service number in order to retrieve the customer details for that service. Reverse telephone directories are used by law enforcement and other emergency services in order to determine the origin of any request for assistance, however these systems include both publicly accessible (listed) and private (unlisted) services. As such, these directories are restricted to internal use only. Some forms of city directories provide this form of lookup for listed services by phone number, along with address cross-referencing. Publicly accessible reverse telephone directories may be provided as part of the standard directory services from the telecommunications carrier in some countries. In other countries these directories are often created by phone phreakers by collecting the information available via the publicly accessible directories and then providing a search function which allows users to search by the telephone service details.

History Printed reverse phone directories have been produced by the telephone companies (in the United States) for decades, and were distributed to the phone companies, law enforcement, and public libraries. In the early 1990s, businesses started offering reverse telephone lookups for fees, and by the early 2000s advertising-based reverse directories were available online, prompting occasional controversies revolving around privacy.

Australia In 2001, a legal case Telstra Corporation Ltd v Desktop Marketing Systems Pty Ltd was heard in the Australian Federal Court. It gave Telstra, the predominant carrier within Australia and the maintainer of the publicly accessible White Pages (residential) and Yellow Pages (commercial) directories, copyright over the content of these directories. In February 2010 a Federal Court of Australia case Telstra Corporation Ltd v Phone Directories Company Pty Ltd determined that Telstra does not hold copyright in the White Pages or the Yellow Pages.

United States In United States, landline phone subscribers can pay a small fee to exclude their number from the directory, depending on the service provider. This service is available as an unlisted number, or an unpublished number. An unlisted number is excluded from public directories. An unpublished number is also excluded from directory assistance services, such as 411. Landline telephone companies often charge a monthly fee for this service. As cellular phones become more popular, there have been plans to release cell phone numbers into public 411 and reverse number directories via a separate Wireless telephone directory. However, these plans have come under opposition from internet based privacy advocate groups, and blogs, often citing privacy concerns. Opposition to the wireless 411 directory was led by consumer advocacy organization Consumers Union. At the time of their campaign; they cited both privacy, and control of out of pocket costs, as reasons for their opposition. In their 2004 congressional testimony supporting the (S. 1963, the "Wireless 411 Privacy Act" 9/2004) bill, Consumers Union claimed that "consumer privacy could be jeopardized by a wireless 411 directory". From 2008 to 2010, Google provided a custom search operator called Google Phonebook which allowed a user to search for a phone number directly within the Google search bar. This feature has since been removed entirely due to privacy concerns. Several information companies provide reverse cell phone lookups that are obtained from various resources, and are available online. Because there is no central database of cell phone numbers, reverse phone directories that claim to be free cannot return accurate information on those numbers, or tend to include only partial data related to the phone number subscriber.

United Kingdom In the United Kingdom proper, reverse directory information is not publicly available. In the Channel Islands it is provided in the printed telephone directories. Although the information is, of necessity, available to emergency services, for other agencies it is treated as 'communication data' in the RIPA regime and subject to the same controls as requests for lists of and content of calls.

References

External links Wayback Machine (21 July 2001) archive of http://blackpages.2600.org.au Federal Court of Australia Case 612 (25 May 2001): Telstra Corporation Limited v Desktop Marketing Systems Pty Ltd

Worked examples

Example 1 — a first encounter with Reverse telephone directory

Start with the simplest possible case. Write down what Reverse telephone directory claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In mathematics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Reverse telephone directory before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Reverse telephone directory ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Reverse telephone directory

In research: Reverse telephone directory appears in mathematics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Reverse telephone directory in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Reverse telephone directory is common in secondary-school and first-year university syllabi. It links to neighbouring topics Directories, Information retrieval systems, Online databases, so understanding it makes those chapters shorter.
In everyday life: Look for Reverse telephone directory outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Reverse telephone directory in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Reverse telephone directory means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Reverse telephone directory out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Reverse telephone directory in simple terms?

Why does Reverse telephone directory matter?

Because it connects several mathematics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Reverse telephone directory?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Reverse telephone directory.