All You Need To Know About Telephony Denial Of Service & Ways to Deal With This Type of Attack

Telephony Denial Of Service

Currently, several businesses are using VoIP as their key communication medium. Cost-effectiveness and functionality benefits are the main reasons behind the growing adoption of VoIP technology. Though there are numerous benefits of using VoIP, like other IT areas, VoIP is also vulnerable to security attacks by hackers, thieves, and miscreants. They can steal important information and disturb the entire business operations. One of the most common VoIP security attacks is the Telephony Denial of Service (TDoS). Though it is not possible to prevent TDoS or other malicious attacks against VoIP systems completely, by implementing effective preventive measures, these attacks can be mitigated or prevented to a great extent. In this blog, we will talk about telephony DOS in detail and the ways how businesses can prevent this type of attack. 

About telephony DoS

A DoS or Denial of Service attack is a practice used by attackers to make a resource unavailable to its intended users. It usually results in a temporary disruption or delay of services. DoS attacks are most common in data systems and networks. However, this type of attack can also occur in a telecom system or network. The industries that suffer the most from TDoS attacks include all types of call centers, government agencies (particularly 911 centers), corporations, banks, etc.

A Telephony Denial of Service or TDoS attack is an attempt by fraudulent parties to make a telephone system inaccessible to the anticipated users by preventing incoming and/or outgoing calls. This happens when the hacker or attacker consumes all the available telephone resources, which leads to no available telephone lines for the intended users. TDoS attacks are generally performed to infuriate and harass a targeted business. 

In the field of IT, TDoS is one of the most common attacks. Hacktivists usually execute these attacks to prevent normal communication with the organization’s systems and services. The attack is successfully done by overflowing the organization’s IT interfaces with huge amounts of data, averting authorized traffic from getting through. They make use of low-cost VoIP tools to execute a huge telephone calling campaign, instantaneously attacking an organization’s telephony services with electronically generated calls. These attacks disturb the organization’s services and prevent them from conducting their business systematically. 

Types of TDoS attacks

There are different types of TDoS attacks. However, they share a common feature of generating several calls to a destination, which ultimately overwhelms the PBX or trunk. These attacks may differ in the way the calls are generated. Usually. TDoS can be categorized as:

  • Centralized TDoS attack
  • Decentralized TDoS attack

In a centralized TDoS attack, hackers generally use computer software to make several calls from one source. 

In a distributed TDoS attack, malware is distributed to numerous call sources to automatically create lots of calls at once.

Both types of TDoS attacks are planned to cut down telephone service. 

Ways to prevent TDoS attack

In the absence of proper security measures, Telephony Denial of Service attacks can overpower important telephone systems including emergency response numbers or call centers. Thankfully, there are various effective solutions available to prevent TDoS attacks. Let’s have a look at some of the best ways to prevent TDoS attacks. 

  • Use trusted IP address: The first and most effective way to prevent TDoS attacks is to use a trusted IP address in the peering SBC. It prevents the system from trying to respond to messages coming from an untrusted or unreliable IP address. This helps organizations to prevent TDoS attacks that send packets to their SBC from the outside of their established network connections.
  • Analyze call traffic: Organizations can also mitigate TDoS attacks by analyzing call traffic. As soon as the volume of traffic breaches a particular limit, they can block the further calls or divert them to other destinations. This way they can easily end the attack. 
  • Use fraud prevention software: Fraud prevention software is another effective measure to prevent TDoS attacks. In the case of centralized attack, the fraud prevention software scores telephone calls with some common attributes like users and groups, calling and called numbers, etc. When the collective score for a common grouping breaches a threshold, the system either blocks the further calls or diverts them to an Interactive Voice Response (IVR) system for a particular period. Thus, ends the attack. In the case of distributed attacks, fraud prevention software scores call traffic to a destination. When the collective score of calls to a given number breaches the threshold, further calls to that particular destination gets diverted to an IVR system for screening purpose. The IVR system asks the caller for a response, which the distributed malware fails to provide. Thus, ending the attack. 


IT organizations must prepare themselves to deal with such types of attacks. The most important part of preparation is to ensure that at the time of the attack, all the uninvited traffic is diverted while allowing the intended users to connect without any interruption. Seeking the help of an expert third-party firm that is well versed in identifying and preventing various attacks including Telephony Denial of Service attacks. These firms can provide the best solutions based on the specific requirements of their customers.

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