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Sok: Everyone Hates Robocalls: a Survey of Techniques Against Telephone Spam SoK: Everyone Hates Robocalls: A Survey of Techniques against Telephone Spam Huahong Tu, Adam Doupé, Ziming Zhao, and Gail-Joon Ahn Arizona State University {tu, doupe, zzhao30, gahn}@asu.edu Abstract—Telephone spam costs United States consumers productivity. A study in 2014 by Kimball et al. [5] found $8.6 billion annually. In 2014, the Federal Trade Commission that 75% of people listened to over 19 seconds of a robocall has received over 22 million complaints of illegal and wanted message and the vast majority of people, 97%, listen to calls. Telephone spammers today are leveraging recent techni- cal advances in the telephony ecosystem to distribute massive at least 6 seconds. Even when the recipient ignores or automated spam calls known as robocalls. Given that anti-spam declines the call, today spammers can send a prerecorded techniques and approaches are effective in the email domain, audio message directly into the recipient’s voicemail inbox. the question we address is: what are the effective defenses Deleting a junk voicemail wastes even more time, taking at against spam calls? least 6 steps to complete in a typical voicemail system. In this paper, we first describe the telephone spam ecosys- tem, specifically focusing on the differences between email Telephone spam are not only significant annoyances, and telephone spam. Then, we survey the existing telephone they also result in significant financial loss in the econ- spam solutions and, by analyzing the failings of the current omy, mostly due to scams and identity theft. According techniques, derive evaluation criteria that are critical to an to complaint data collected by the FTC, Americans lose acceptable solution. We believe that this work will help guide more than $8.6 billion due to fraud annually, and the vast the development of effective telephone spam defenses, as well as provide a framework to evaluate future defenses. majority of them (and still increasing) are due to phone communication [4]. This situation is surprising, given the significant gains made in reducing the amount of email I. INTRODUCTION spam. This raises the question: are there any simple and The national and global telephony system is a critical effective solutions that could stop telephone spam? The un- component of our modern infrastructure and economy. In fortunate answer is no. We found that this issue is not easily the United States (US), the mobile telephone subscribership solved, and, in fact, the simple and effective techniques penetration rate has already surpassed 100% [1]. According against email spam cannot be applied to telephone systems. to the U.S. Bureau of Labor Statistics, each day more than There are significant differences and unique challenges in the 240 million hours are spent on telephone calls in the United telephone ecosystem that require novel approaches. Many States, equating to more than 88 trillion hours each year [2]. existing solutions have failed to overcome these challenges However, with the pervasiveness of telephone service and, as a result, have yet to be widely implemented. subscribership, telephone spam has also become an increas- The objective of this paper is to survey the existing ingly prevalent issue in the US. Recent technical advances solutions in combating telephone spam and, by analyzing the in the telephony ecosystem are leveraged by spammers to failings of the current techniques, derive the requirements distribute massive automated spam calls, known as robo- that are critical to an acceptable solution. This work will calls. The Federal Trade Commission’s (FTC) National Do help guide the development of effective telephone spam Not Call Registry’s cumulative number of complaints of defenses, as well as provide a framework to help evaluate illegal calls in the US totaled more than 22 million in the techniques against telephone spam. 2014 [3], with about 200,000 complaints each month about robocalls alone [4]. Despite US laws prohibiting robocalling The main contributions of this paper are the following: and telephone spamming (with some exceptions), complaints • We describe the telephone spam ecosystem, focusing on illegal calls have reached record numbers year after on the players involved and the technical challenges year, which indicates that the laws have not deterred the that make telephone spam distinct from email spam. spammers. • We develop a taxonomy that classifies the existing anti- Spam calls are significant annoyances for telephone users. spam techniques into three categories, providing a high- Unlike email spam, which can be ignored, spam calls level view of the benefits and drawbacks of each type demand immediate attention. When a phone rings, a call of technique. recipient generally must decide whether to accept the call • We provide a systematization of assessment criteria and listen to the call. After realizing that the call contains for evaluating telephone spam countermeasures, and unwanted information and disconnects from the call, the we evaluate existing techniques using these assessment recipient has already lost time, money (phone bill), and criteria. • We provide a discussion on what we believe to be the Long Distance Network Interexchange Carrier Interexchange Termination Network Termination future direction of solving the telephone spam problem. Termination Carrier Open Internet Open VoIP CarrierVoIP Trunk Line II. BACKGROUND While email spam is arguably the most well-known form Spammer Victims of spam, telephone spam is now more popular than ever. The Public Switched Telephone Network (PSTN) is an Possibly further anonymized behind VPNs and Possibly routing through more layers than aggregate of various interconnected telephone networks that Tor networks depicted adheres to the core standards created by the International Figure 1: Routing of a spam call. Telecommunication Union, allowing most telephones to intercommunicate. We define telephone spam as the mass VoIP Interexchange Termination distribution of unwanted content to modern telephones in the Victim Spammer Carrier Carrier Carrier PSTN, which includes voice spam that distributes unwanted voice content to answered phones, and voicemail spam that distributes unwanted voice content into the recipient’s Leads Seller voicemail inbox. Due to the much greater capacity of IP infrastructure Figure 2: The flow of money in the telephone spam ecosys- and the wide availability of IP-based equipment, telephony tem. service providers have shifted their network infrastructure to IP-based solutions, and the operation cost of the telephone network has dramatically decreased. While the core PSTN than email spam, particularly when spammers use techniques infrastructure has evolved to be almost entirely IP-based, such as caller ID spoofing. the core signaling protocols have not changed. The entire A. Key Players of Telephone Spam ecosystem still relies on the three-decade-old Signaling Sys- tem No. 7 (SS7) [6] suite of protocols, allowing any phone to To understand the telephone spam ecosystem, we will first reach any other phone through a worldwide interconnection identify and explain the roles of all players who take part in of switching centers. the routing of a telephone spam. Figure 1 show a graph- A very common way of disseminating telephone spam ical depiction of the routing process: The spammer con- is robocalling, which uses an autodialer that automatically nects through the Internet to an Internet Telephony Service dials and delivers a prerecorded message to a list of phone Provider, then the call is routed through an Interexchange numbers. An autodialer is a generic term for any com- Carrier, before finally being accepted by the Termination puter program or device that can automatically initiate calls Carrier, who then routes the call to the victim. to telephone recipients. Today, an autodialer is usually a Another way to understand the ecosystem is to show computer program with Voice over Internet Protocol (VoIP) how money flows through the system, which we display in connectivity to a high volume VoIP-to-PSTN carrier, that Figure 2: the money flows from the victim to the spammer, may include features such as voicemail and SMS delivery, and the spammer uses this money to obtain leads (new phone customizable caller ID, Call Progress Analysis, scheduled numbers to spam) and to pay for the spam calls, the Internet broadcast, text-to-speech, Interactive Voice Response, etc. Telephony Service Provider receives the money from the The high reachability of telephone numbers has led to spammer and pays the Interexchange Carrier, who then pays telephony being an attractive spam distribution channel. Al- the Termination Carrier. Next we examine each of these most every adult in the US can be reached with a telephone roles in turn. number, and the vast majority of telephone numbers are Spammer is the agent that carries out the spamming mobile telephone subscribers. Although VoIP usage has been operation. The spammer could be part of an organization, growing rapidly, we found that it is more of an add-on or an independent contractor that offers spamming-as-a- protocol (instead of a wholesale replacement) of existing service. The goal of the spammer is usually to extract mobile wireless and landline services. Using 2013 statistics, money from victims through sales and scams, or to launch there are about 335 million mobile telephone subscribers [1], a campaign of harassment. For cost efficiency, spam calls 136 million fixed-telephone subscribers [7], and 34 million are typically initiated using an autodialer connected to an VoIP subscribers [8] in the US (population 318 million). Internet Telephony Service Provider to reach the PSTN We believe the improved cost efficiency of telephone victims. Currently, spamming to VoIP victims are not as spamming, advancement of spam distribution technology, common, mainly due to the limited pool of potential victims, and high reachability of telephone numbers contributed to and some VoIP users, such as Skype, may not be reachable the recent surge in telephone spam. Furthermore, we believe most of the time. We will describe the spammer’s operation that telephone spam has the potential to be more persuasive in more detail in Section II-B.
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