Imagine you’re talking on the phone, but instead of using a traditional phone line, your voice travels through the internet. That’s what VoIP (Voice over Internet Protocol) does! It’s super important for businesses because it can save money and add cool features like video calls.  

But, even with great technology, there’s something called a firewall that keeps our internet and VoIP safe. Sometimes, it gets a bit too protective and can cause problems with VoIP calls. Let’s dive into what these issues are and how we can fix them. 

What is a firewall and how does it work? 

osi model 7 layers

A firewall acts as a security guard for your computer or network, deciding which data packets can enter or leave based on specific rules. It’s helpful to think of your network as a private club, with the firewall as the bouncer at the door, checking each guest (data packet) against a guest list (set of rules) before deciding who gets in. 

To understand how firewalls work, we need to dive a bit into how data is sent over networks, using something called the OSI (Open Systems Interconnection) model. This model has several layers that explain how data travels from one device to another. The most important layers for firewalls are: 

  • The Network Layer: This is like the address on an envelope. It tells the firewall where the data packet is coming from and where it’s going, using something called IP addresses. 
  • The Transport Layer: This layer is like specifying whether you want your letter sent by regular mail, express, or registered. It deals with how the data gets to its destination, using protocols named TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). These protocols use port numbers to direct the data to the right application, like sending a letter to a specific department in a large office building. 

When a data packet arrives at the firewall, it looks at this information – the IP addresses from the network layer and the TCP/UDP port numbers from the transport layer.  

The firewall uses this information to check against its rules. If the rules say the data packet is okay, it lets it through; if not, it blocks it. This is like a security guard checking your ID and invitation before letting you into a government building. 

Firewalls are usually placed at the entrance of a network, like the main door of a building, to check data packets as they come in and go out. This strategic placement ensures that only safe and authorized data can move in and out of the network, protecting the devices and information inside from unauthorized access or harmful attacks. 

A firewall works by using detailed information in data packets to enforce security rules, acting as a critical checkpoint that guards the entrance to your digital space. 

Firewall Types 

Firewalls have evolved significantly since their inception, transitioning from basic network security functions integrated into routers to sophisticated stand-alone devices designed to protect networks from a wide range of cyber threats.  

The evolution of firewall technology can be understood through its generational changes, each bringing new capabilities and enhancements over its predecessors. Here’s an exploration of the different types of firewalls that have been developed over time: 

1. Packet Filtering Firewalls (First-Generation) 

The first wave of firewall technology consisted of packet-filtering firewalls. These devices operate by examining the headers of data packets as they attempt to pass through the network.  

They scrutinize source and destination IP addresses, along with TCP or UDP port numbers, against a predefined set of rules. If a packet’s details match the allowed criteria, it passes through; otherwise, it’s blocked. This method is straightforward and effective for basic filtering but lacks the depth to inspect the actual content of the packets or understand the context of data traffic. 

2. Stateful Inspection Firewalls (Second-Generation) 

Building upon packet filtering, the next advancement came with stateful inspection firewalls. These firewalls retain the capabilities of their predecessors but add the ability to monitor and understand the state of active connections.  

By keeping track of ongoing conversations between endpoints, stateful firewalls can make more informed decisions about which packets should be allowed or blocked. They assess whether incoming traffic is part of an established connection or unsolicited, enhancing the security by adding context to the decision-making process. 

3. Application Layer Firewalls (Third-Generation) 

As cyber threats became more sophisticated, firewalls adapted by extending their protective measures to the application layer, known as Layer 7 in the OSI model. These application layer firewalls, or third-generation firewalls, are equipped with deep packet inspection (DPI) capabilities.  

DPI allows these firewalls to examine the content within data packets, identifying the specific application protocols being used, such as HTTP, FTP, SIP, and RTP. This granular inspection enables the firewall to enforce security policies based on the nature and behavior of the application traffic, rather than just IP addresses and port numbers.  

Configuring these firewalls requires a higher level of detail, as they need to understand the applications’ logic and behavior to effectively filter malicious or unwanted traffic. 

VoIP and Firewall 

Firewalls, essential for data security, can cause issues with VoIP due to their complex interaction with VoIP protocols like SIP for call control and RTP for voice packet transmission.  

The added layer of network address translation (NAT) by firewalls further complicates VoIP communication, affecting call quality and reliability. VoIP systems, involving multiple communication channels, require careful firewall configuration to ensure seamless and secure voice communications. 

Problems with Firewall Affecting VoIP Performance 

Firewalls can disrupt VoIP communications by treating related sessions as separate, leading to issues like one-way audio or total communication loss.  

This problem persists in various VoIP setups, including calls over SIP trunks or between branch sites connected via WAN, as they all must navigate through firewalls, which may not consistently recognize and allow all necessary VoIP traffic components. 

What are the best practices when implementing VoIP with Firewall? 

For optimal VoIP performance alongside firewalls, consider the following best practices: 

  • Avoid first-generation firewalls due to their limited session management capabilities for VoIP. 
  • Use at least a stateful firewall to better manage VoIP sessions. 
  • Prefer application layer firewalls for deeper packet inspection to ensure VoIP traffic integrity. 
  • Install a Session Border Controller (SBC) for enhanced VoIP communication management and security. 
  • Disable SIP ALG on firewalls to prevent potential VoIP disruptions. 
  • Transition to IPv6 to overcome NAT issues and enhance VoIP security

These practices help maintain security without compromising VoIP functionality. 


We talked about how firewalls can sometimes mess with VoIP calls and what we can do about it. By staying on top of settings and using the right tools, we can keep our internet calls smooth and secure. With expert business phone system provider like RingOffice helping out, businesses can make sure their VoIP systems work perfectly.

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