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Caller ID: maintaining investigative security.

The telephone has become such a staple of modem life that few people give it a second thought. When callers pick up the receiver, it is doubtful they consider the millions of signals being routed through switching stations that their call is about to join. They just know that when they want to check in with a family member across town or a business associate across an ocean, they only need to pick up the telephone. Even when power goes out in a community, the telephones generally continue to work. So, it might be easy to take this workhorse of the information age for granted.

However, advances in telephone service options - most notably caller identification services - require that law enforcement agencies take a close look at how they use the telephone. The growing prevalence of caller identification services (generally referred to as caller ID) dictates that investigators take special precautions, especially during undercover operations.

Caller ID: Help or Hindrance

As its name implies, the caller ID device displays the originating telephone number of an incoming call, allowing the recipient to know, before answering the call, the number of the party calling.

For law enforcement, caller ID has proven to be a valuable intelligence tool. When investigators install a court-authorized wiretap or dialed number recorder on a telephone line, for instance, they also generally request caller ID. With caller ID on the line, investigators not only know whom the targeted subject calls but also who calls the subject.

Investigators also can include a suspect's caller ID device on a search warrant request. A properly worded search warrant allows investigators to seize the caller ID box and thus obtain an accurate record of the last 25 to 100 calls received by the subject.

Nevertheless, despite its benefits, caller ID poses some potentially serious problems for the police. Critics claim that it invades citizens' privacy. There are also concerns that caller ID may reduce the number of calls to police crime tip lines, crisis centers, and suicide and abuse hotlines.

For law enforcement agencies, concerns primarily revolve around the effects caller ID and related services have on undercover operations. By understanding the functions of these services, however, investigators can develop strategies to maintain telephone security during investigations.


Caller ID comes in two forms. Basic caller ID (sometimes referred to as single message) represents the first generation of caller identification services, widely available since the early 1980s. During the last several years, telephone companies have been converting to enhanced caller ID (also known as deluxe or multimessage).

The primary difference between the two systems is the amount of information provided about the originating telephone call. While basic service provides only the caller's telephone number and the date and time of the call, enhanced service supplies this information, as well as the name and in some cases, the address of the caller.

Regardless of which form of caller ID serves a particular locality, the mechanics of its operation remain the same. The local telephone company attaches caller ID at its central office after the originating call has been placed. This makes it nearly impossible for the caller to trick or defeat the system.

Once the caller ID codes have been attached, the caller's identifying information is routed on the line with the call itself to the destination telephone. Caller ID information reaches the receiving telephone between the first and second rings. Ira call is answered before or during caller ID delivery, the answering party will not receive the data.


If a caller has installed call blocking - an optional service to prevent transmission of the originating telephone number and other identifying information - this request is attached at the central telephone office after the commands for caller ID have been attached. When the call reaches the central office for the area serving the destination telephone, the office handles caller ID according to local programming. If the party at the destination telephone has paid for caller ID services, identifying information from the originating call will be sent.

If a call blocking command has been attached by the party making the call, the call will go through but the identifying information will not be relayed. Instead, a message indicating that all identifying information has been blocked will accompany the call. Generally, the word "private" or some variation appears on the caller ID screen, notifying the recipient that the caller has concealed the originating telephone number.


Newly relaxed regulations and advances in technology soon will make caller ID and related services available on a much larger scale. Until recently, regional telephone companies dictated local service availability. Often, parties with caller ID would receive an "out of area" message, indicating that an incoming call was being placed from a locality that did not relay caller identification information.

In December 1995, the Federal Communications Commission allowed caller ID services to be relayed nationwide. As telephone companies gradually expand service availability, caller ID will become a truly national system. Already, there are indications that caller ID will be offered on a worldwide basis in the not-too-distant future.

Rapidly advancing technology also has enabled carriers to offer caller ID services on calls originating from sources that were once immune, including cellular and pay telephones. As with calls from localities that do not pass caller ID, calls from these types of telephones previously would relay an "out of area" message. Now, calls placed from cellular or pay telephones, as well as long-distance calls paid for via credit or phone cards, may provide identifying information to the party being called.

In fact, some firms that specialize in emerging technologies heavily promote their caller identification capabilities. The newest competition to cellular service, personal communications systems (PCS), pass identifying information in both directions. A screen on the handset lets users know the originating telephone number of the party calling them. Likewise, callers using PCS will pass on their identifying information to anyone with a PCS unit or caller ID. PCS users do not need to activate caller ID service separately; the caller identification features are included in the basic service contract.

With expanding caller identification services, law enforcement agencies should study the various methods available to respond to the threats posed to undercover investigations. Because no single antidote exists for every situation, investigators should be aware of the broad range of possible countermeasures to caller ID.


Given the dramatic growth of caller ID and the impracticality of determining whether an individual has this service option before a call is placed, investigators should assume every subject can identify them when they call. Although caller ID cannot be defeated after a call is placed, investigators can minimize its effects.

Call Blocking

While placing call blocking on the originating telephone line may be the most obvious countermeasure, certain features of this service make it less than practical for undercover operations. As discussed, a message will alert the party on the receiving end that the caller has placed a block on the outgoing line. Such a signal could further inflame the suspicions of an already wary subject.

Then, because call block commands are attached to a call after the caller ID commands, investigators must gamble that the commands - sometimes routed by two different telephone companies - are attached properly. If not, a call from the police station could be routed unblocked to a subject. While such mishaps are extremely rare, just one could prove disastrous.

In response to widespread concerns about privacy issues surrounding caller ID, telephone companies have developed more specialized call blocking features. Per-call blocking defeats delivery of caller ID on a call-by-call basis; per-line blocking defeats caller ID on a specific outgoing telephone line.

In most areas where these features have been introduced, however, the local telephone carrier also offers service options to counter call blocking. The anonymous call rejection feature automatically routes calls with call blocking to a recording that advises callers to dial again without blocking caller ID.

Some state and local governments have arranged with telephone companies to provide call blocking features only for government lines. Still, whether in a general form or on a per-call or per-line basis, call blocking might not provide the stealth necessary for undercover operations.

If, however, a law enforcement agency decides to use call blocking in any of its forms, investigators must be careful to ensure that it is attached properly. In some areas, telephone companies use the same code to block caller ID as they do to cancel the block. This can become an especially confusing issue with per-line blocking. When investigators enter the blocking code on a particular line, the end result actually may be to reactivate caller ID on a line where it previously had been blocked. To avoid such scenarios, investigators should not rely on call blocking.

Credit or Phone Cards

While placing calls using a credit or phone card has, in the past, been a fairly safe way to defeat caller ID, investigators cannot assume that these tactics will continue to work every time. An increasing number of telephone companies have begun to relay some type of identifying information via calls placed with credit and phone cards. Telephone companies also may periodically test new features, intermittently relaying identifying information on a limited number of calls before fully engaging a system.

It is increasingly dangerous for investigators to assume that a credit or phone card call will not betray their identities to parties with caller ID. Recent reports indicate that some calls routed by a particular carrier would display "U.S. Government" when placed with a government credit card. Revelations of this type pose obvious dangers both to investigations and investigators.

Pay Telephones

Investigators should remember the limitations of using pay telephones to defeat caller ID. Today, most pay phones relay caller ID information. Industrious subjects can use databases (available to the public) that show the location of each pay phone in a given city to pinpoint a caller's location. One investigator described a case where he called a subject from a pay telephone. While they were still talking, the subject tracked the investigator's position and went to the phone booth shortly after the call.

Investigators should be aware that although a call placed from a pay telephone will not show the caller's name and address, it may show the caller's location. A call placed from a phone booth outside the police station or federal building may be just as disruptive to a case as a call placed from the office telephone.

Undercover Lines

Agencies can use telephone lines dedicated solely to undercover operations. However, regardless of what name is on file, the telephone company will maintain a record of the physical location of the telephone. To enhance security, agency administrators should work with telephone company officials to ensure that the billing address does not reflect a law enforcement connection. The computer containing the wiring information still will connect to the actual location of the telephone, no matter where the bill is sent.

Call Diverters

A device called a call diverter represents one of the more effective countermeasures to caller ID. These devices forward outgoing calls from one telephone line to another, effectively masking the identity of the original caller. By enabling agencies to forward calls at the source, these devices offer more security than call forwarding and other services available from the telephone company.

Units cost between $400 and $5,000, depending on the features included, but may prove well worth the investment for agencies that engage regularly in undercover operations. Investigators should be able to obtain detailed information about call diverters from their agency's electronic surveillance support unit.

Testing Caller ID Availability

Regardless of what technical countermeasure investigators employ to defeat caller ID, they should periodically make test calls from the telephone they normally use for undercover operations to monitor how caller ID is being handled in a particular area. By calling a telephone equipped with caller ID, investigators can determine what type of identifying information is being relayed via the telephone lines. Calling the local telephone company is not a good indicator; customer service representatives do not always know what specific information is relayed at any given time because, among other reasons, technicians frequently activate new features for testing purposes. However, even conducting regular tests does not ensure security because a subject may have different caller ID capabilities than the investigator's test line.


Like much of the general population, many investigators might be unfamiliar with automatic number identification (ANI). Intended for use primarily by businesses, ANI provides subscribers with a wealth of identifying information concerning callers. As is the case with caller ID, this service option poses potential security problems for investigators.

Despite its obscurity, ANI actually predates caller ID. The systems serve similar purposes, but unlike caller ID, ANI coding is sent on a separate wire, rather than with the call itself. ANI is available only on numbers beginning with either 800, 888,900, or 911 or a 976 exchange. Businesses routinely use ANI to gather information on their callers for billing purposes. There is no way to block ANI delivery.

In the wrong hands, ANI can provide criminals with personal information about anyone placing a call to them. During a recent case, a law enforcement officer used an 800 number to access a subject's personal pager. The officer punched in a telephone number different from the one he was calling from for the subject to call. But, because the subject's pager service tracked ANI data and forwarded it to subscribers, the subject was able to tell the officer the number of the telephone from which he originally called.

Because telephone companies do not offer services to counteract ANI, law enforcement agencies must rely on alternate methods to safeguard security. While call diverters represent the most effective countermeasure against ANI, given their cost and impracticality for use during many fast-paced investigative scenarios, investigators cannot rely on them to ensure security for every situation.


While it might be easy to take the telephone for granted, law enforcement agencies cannot afford to become complacent about telephone security. Evolving caller ID services represent a potentially serious threat to undercover operations for law enforcement agencies in an increasing number of communities around the country.

By developing a flexible array of countermeasures, agencies can minimize the dangers posed by caller ID. Investigators must remember that no countermeasure can be guaranteed effective for every situation. Instead, they should take precautions and be prepared for any problem that might arise from breaches of security due to caller ID. After all, the security of law enforcement operations is on the line.

Mr. David P. Williams serves with the Electronic Surveillance Unit, Office of Investigations, at the U.S. Customs Service headquarters in Woodbridge, Virginia.
COPYRIGHT 1997 Federal Bureau of Investigation
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1997, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Williams, David P.
Publication:The FBI Law Enforcement Bulletin
Date:Oct 1, 1997
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