The Complete Guide to How SIP Trunking Works

Posted on March 25, 2024

SIP Trunks: The Quick Definition

SIP trunks, short for Session Initiation Protocol trunks, transforms how we handle voice and various forms of communication, moving them over the internet rather than using old-school phone wires. This shift is crucial for developers and IT leaders, offering a pathway to more agile, scalable, and budget-friendly communication options.

In our digital age, finding reliable and efficient ways to communicate is a top priority for businesses. Enter SIP trunks—a game-changer in the realm of global business communication. It’s especially vital for developers tasked with adding voice or messaging capabilities to apps, and for IT leaders in small businesses making the switch from old-school telephony to VoIP, all without causing a hiccup in day-to-day activities. SIP trunks cater to the modern need for agile, internet-based communication solutions. It’s perfectly in step with the market’s movement, too. The SIP trunk services market is experiencing swift growth, underscoring its growing significance in today’s business environment.

How SIP Trunks Transform Business Communications

SIP trunks changes the game by swapping old phone lines with the internet for sending voice, video, texts, and more. This shift cuts costs big time while giving unmatched freedom and room to grow. For developers, it’s a dream come true – a simple way to add voice and texting to apps. Imagine making your client’s system smarter with VoIP or juggling all kinds of chats and calls remotely. SIP trunking makes it all smooth and solid.

Importance of SIP Trunks: At its core, SIP trunks encapsulates the future of business communications. Its importance can’t be overstated as it enables seamless connectivity in a decentralized work environment, offering a robust framework for unified communications. For developers and IT leaders, leveraging SIP trunks means ensuring that their applications or business systems are not only ready for today’s demands but are also future-proof.

Infographic detailing SIP Trunking process: 1. Initiation of a call over the internet. 2. SIP trunk connects call to the recipient's network. 3. Conversion of digital signals to voice. 4. Delivery of voice call. 5. Disconnection upon call end. Key Benefits: Cost savings, scalability, enhanced feature set, and reliability. - how sip trunk works infographic infographic-line-3-steps

As we delve into this guide, keep in mind that SIP trunks are more than just technology; it’s a strategic move towards embracing digital transformation in your communication infrastructure.

Understanding SIP Trunking


SIP trunking is a method of sending voice and other communication services over the internet. It works with SIP to initiate, maintain, and terminate real-time sessions that include voice, video, and messaging applications. This technology is key to enabling VoIP (Voice over Internet Protocol), which allows for calls to be made over the internet instead of traditional telephone lines.

How SIP Trunks Replace Traditional Telephony

In the past, businesses relied on physical phone lines (like those of the Public Switched Telephone Network, or PSTN) to make and receive calls. This meant installing multiple lines for different extensions, which was costly and inflexible. SIP trunks changes the game by using the internet to route these calls, eliminating the need for physical lines. This not only reduces costs but also simplifies the telephony infrastructure, making it easier to manage and scale.

Benefits for Businesses

  • Cost Savings: Traditional phone lines often come with long-term contracts and fixed costs, regardless of how much or how little you use them. With SIP trunks, businesses can often pay for what they use, leading to significant cost savings. For example, domestic inbound calls can cost a fraction of a cent per minute, which is much lower than traditional carriers.
  • Scalability: As your business grows, so does your need to communicate. SIP trunks allow for easy scaling of communication channels without the need for additional physical infrastructure. Whether you’re adding new employees or expanding to new locations, SIP trunks grow with your business. This scalability is crucial for companies looking to expand their remote or distributed workforces.
  • Flexibility: SIP trunks work with a variety of communication devices, including mobile phones, software phones, IP phones, and compatible private branch exchanges (PBXs). This means businesses can use their existing devices, making the switch to SIP trunks smoother and allowing employees to work from anywhere with a stable internet connection.
  • Reliability: With traditional telephony, a single point of failure can disrupt your entire system. SIP trunks, provided over the internet, offers dynamic rerouting of calls in case of an outage, ensuring that your business communications remain uninterrupted. Some cloud-based SIP providers boast uptimes greater than 99.99%, providing peace of mind for businesses that rely heavily on voice communication.

SIP trunking is not just a technological upgrade; it’s a strategic investment towards modernizing your company’s communication framework. It offers a blend of cost efficiency, scalability, and reliability that traditional telephony systems simply can’t match. For developers and IT leaders, understanding how SIP trunks work is the first step in leveraging its full potential to drive business growth and efficiency.

Technical Components of SIP Trunking

SIP Protocol

At the heart of SIP trunking is the SIP. It’s a set of rules that computers follow to communicate with each other over the internet. Think of it as the language that devices use to start, run, and end calls. SIP is crucial because it can support voice calls, video conferencing, and instant messaging all within the same framework. This versatility is what makes SIP so powerful for modern communication systems.


In the traditional sense, a trunk is a line or link that can carry many signals at once, connecting major switching centers in a communication system. In SIP trunking, a SIP trunk is the virtual version of this. It’s the digital pathway through which all your internet-based calls travel. Unlike the old phone lines that needed physical wires, a SIP Trunk can handle thousands of calls at once – all you need is a good internet connection.


An IP-PBX (Internet Protocol Private Branch Exchange) is a private telephone network used within a company. The IP-PBX is a crucial component because it routes calls between users on local lines while allowing all users to share a certain number of external phone lines. The main difference between a traditional PBX and an IP-PBX is that the latter uses the Internet Protocol to manage calls, making it more flexible and easier to integrate with SIP trunks.

VoIP Gateways

For businesses that still have traditional phone systems but want to take advantage of SIP trunking, VoIP Gateways are the answer. A VoIP Gateway acts as a bridge between the old (PSTN) and the new (SIP trunk). It converts digital call data from SIP into analog signals for the PSTN and vice versa. This way, companies can keep their existing infrastructure while enjoying the benefits of internet-based calls.


Lastly, codecs are what make digital communication possible. They compress your voice into digital packets that can travel over the internet and then decompress it back into voice at the other end. There are many different codecs, each with its own balance of sound quality and bandwidth usage. Choosing the right codec can significantly affect the clarity of your calls and the efficiency of your SIP trunk setup.

How SIP Trunking Works: A Step-by-Step Process

Let’s dive into the core of how SIP trunk works in a step-by-step fashion. This will help developers and IT leaders understand the process, making it easier to implement and troubleshoot within their organizations.


The journey of a SIP trunk call begins with Initialization. This is where the call setup starts. A device, like an IP phone or a softphone application, requests to start a communication session. It sends a SIP INVITE message to the SIP server. This message contains vital information about the session, such as the caller and receiver’s addresses.


Authentication is the next critical step. The SIP server checks if the device attempting to start the call is authorized to do so. This involves verifying credentials like usernames and passwords. It’s a security measure to ensure that only legitimate users can make calls. Once the server confirms the credentials are correct, it proceeds to the next step.

Session Establishment

After authentication, we move to Session Establishment. The SIP server contacts the receiver’s device, conveying the call request. If the receiver’s device is ready and accepts the call, it sends a 200 OK response back to the server. This back-and-forth communication establishes the session parameters, like which codecs will be used for the call. This step ensures both ends can communicate effectively.

Data Transfer

With the session established, Data Transfer can begin. Voice, video, or any other form of data starts flowing between the caller and receiver. This is the step where the conversation happens. The data is broken into packets and transmitted over the internet or through the PSTN, depending on the network setup. This process is seamless and continuous until one party ends the call.


Finally, we have Termination. When the call is finished, either party can initiate the termination. A SIP BYE message is sent, signaling that the session should be closed. Both devices acknowledge the end of the session, and the call is officially over. The resources that were allocated for the call are then freed up, ready to be used for the next communication session.

This step-by-step process of how SIP trunk works showcases the efficiency and reliability of SIP trunks as a communication solution. From the initiation of a call to its termination, every step is designed to ensure secure, high-quality communication. For developers and IT leaders, understanding this process is crucial for implementing, managing, and optimizing SIP trunk solutions in their organizations.

Advantages of SIP Trunks for Developers and IT Leaders

SIP trunks are efficient, scalable and cost-effective. For developers and IT leaders steering the digital transformation in their organizations, the transition to SIP trunks is not just a step but a leap towards operational excellence. Let’s dive into the core advantages that SIP trunks offer.

Cost Savings

One of the most compelling reasons to adopt SIP trunks is the significant cost reduction it brings. Traditional phone lines often involve hefty initial investments and ongoing expenses. SIP trunks, on the other hand, operates over the internet. This means you can say goodbye to the costs associated with physical lines. Moreover, the pay-as-you-go model allows businesses to only pay for what they use. Flowroute highlights that domestic inbound calls can cost a fraction of a cent per minute, showcasing the undeniable cost-effectiveness of SIP trunking.


Growth is an essential aspect of any business, and SIP trunks align perfectly with this goal. Whether you’re scaling up or down, SIP trunks adapt to your needs without requiring significant investment in new infrastructure. This scalability ensures that businesses can manage their communication needs efficiently, accommodating remote or distributed workforces with ease. The ability to scale on demand is a game-changer, especially in today’s dynamic business environment.


Flexibility in communication is another standout benefit. SIP trunks enables businesses to manage their phone systems with agility, integrating with existing IP-PBX systems or leveraging the cloud for advanced features. This flexibility extends to how calls are made and received, allowing users to communicate from anywhere in the world. For developers, this means you can design systems that cater to the modern workforce’s needs, ensuring seamless communication regardless of location.


In business communication, reliability is non-negotiable. SIP trunks deliver high-quality voice service through VoIP, ensuring that calls are routed to the fastest path over the internet. With dynamic failover capabilities, SIP trunk providers like Flowroute offer uptime greater than 99.99%, ensuring that businesses remain connected even in the face of network issues or outages. This level of reliability is crucial for maintaining business continuity and customer satisfaction.

Enhanced Features

SIP trunks are not just about making and receiving calls. It offers a suite of enterprise-grade features that can elevate business communication. Features such as remote call forwarding, CNAM (caller ID name), and phone number masking are just the tip of the iceberg. For developers, this opens up a world of possibilities to create customized communication solutions that cater to specific business needs. Whether it’s integrating voice and messaging into software applications or leveraging APIs for seamless connectivity, SIP trunks provides the tools needed to innovate.

Implementing SIP Trunks in Your Organization

When you’re ready to bring SIP trunks into your organization, it’s crucial to approach the process methodically. This ensures a smooth integration with minimal disruption. Let’s break down the steps: Assessment, Choosing a Provider, Configuration, Testing, and Deployment.


First, take a detailed look at your current telecommunication setup. Identify what you have – maybe a legacy PBX or an existing VoIP system. Also, consider your future needs. How many calls do you expect to handle? What kind of growth are you anticipating? This step is about understanding your starting point and where you want to go.

Choosing a Provider

Choosing the right SIP trunk provider is essential for the success of your business’s communication system. It’s important to select a provider that prioritizes reliability and matches your unique requirements. Search for providers that make it simple to test their services, ensuring they meet your standards for quality and integration. Key factors to consider include the provider’s ability to scale with your business, the level of customer support they offer, and their pricing structure. A careful evaluation of these aspects will help you find a SIP trunk provider that supports your business effectively.


Once you’ve chosen a provider, it’s time to configure your system. This involves setting up your IP-PBX to connect with your SIP trunk. If you’re using a software-based solution, you’ll likely have a SIP gateway included. However, with a legacy system, you may need a separate gateway device. This step is crucial and requires a good understanding of both your PBX and the SIP trunking service. Resources such as The SIP School can be invaluable for gaining the necessary expertise.


Before going live, thoroughly test your setup. Make calls within your network and to external numbers, checking for any issues with call quality, connection times, or dropped calls. This phase is about ensuring everything works as expected and making adjustments as needed. It’s better to catch and fix problems now than after deployment.


With testing complete and any issues resolved, you’re ready to deploy your SIP solution. Start by transitioning a small portion of your traffic and monitor performance closely. This allows you to address any unforeseen challenges without affecting your entire operation. Gradually increase the volume of SIP-based communications as your confidence in the system grows.

Common Challenges and Solutions

When deploying SIP, developers and IT leaders might encounter a few hurdles. However, with the right approach, these challenges can be easily overcome. Let’s explore some of the common issues and their solutions.

Compatibility Issues

The Challenge: One of the first roadblocks you might face is compatibility between your existing systems and the SIP trunking service. This can include issues with IP-PBX systems, VoIP gateways, and even the phones themselves.

The Solution: It’s crucial to check that all your hardware supports SIP technology, aiming for devices that are compliant with SIP RFC 3261 standards. This may require updating your devices’ software or possibly replacing old equipment with newer, compatible options. Choosing providers that accommodate a broad spectrum of hardware can greatly ease this transition.

Security Concerns

The Challenge: Security is a major concern when it comes to SIP trunks. Threats like toll fraud, eavesdropping, and DDoS attacks can pose significant risks to your communication infrastructure.

The Solution: To protect your SIP trunk setup, adopt strong security measures. Start with robust authentication methods and encryption to safeguard your data and communications. A secure network configuration is also vital to prevent unauthorized access.

A specialized firewall plays a critical role in this security framework. Ensure it’s correctly set up to manage SIP traffic effectively, without leaving gaps in your defense. This involves allowing legitimate calls to pass through while blocking potential threats.

Adhering to best practices for SIP trunk security is another key step. These include regularly updating software and firmware to patch vulnerabilities, using complex passwords, and restricting account access based on IP addresses. By implementing these strategies, you can create a resilient barrier against cyber threats, keeping your telecommunications secure.

Quality of Service

The Challenge: Dropped calls, one-way audio, and poor call quality are common issues that can affect the user experience. These problems are often related to network configurations, such as NAT issues, or insufficient bandwidth.

The Solution: Prioritize SIP and VoIP traffic on your network to ensure it gets the bandwidth it needs, especially during peak usage times. Configuring Quality of Service (QoS) settings on your network can help manage traffic effectively, ensuring high-quality voice transmission. Regularly monitoring and testing your network’s performance can also identify and rectify issues before they impact users.

Bandwidth Requirements

The Challenge: SIP trunks require sufficient bandwidth to handle concurrent calls without degradation in quality. Underestimating bandwidth needs can lead to poor call quality and dissatisfaction among users.

The Solution: Calculate your bandwidth needs based on the maximum number of concurrent calls you expect, factoring in the codec used for compression and the overhead for signaling. As a rule of thumb, a single VoIP call using the G.711 codec requires approximately 85-100 Kbps of bandwidth (up and down). It’s also wise to have a buffer to accommodate unexpected spikes in call volume. Upgrading your internet connection may be necessary if your current bandwidth is insufficient.

By understanding and addressing these common challenges, developers and IT leaders can ensure a smooth and successful SIP trunk deployment. This proactive approach not only solves immediate issues but also sets the stage for a scalable, secure, and high-quality communication system that meets the needs of modern businesses.

Are SIP Trunks Right for You?

The landscape of business communications is rapidly evolving, and SIP trunks are at the forefront of this transformation. As we look to the future, it’s clear that SIP trunks will continue to play a pivotal role in how organizations communicate, both internally and with their customers. The flexibility, scalability, and cost-effectiveness of SIP trunks make it an indispensable tool for businesses aiming to stay competitive in a digital world.

With technological advancements, we can expect SIP trunks to become even more robust, offering enhanced features, better security, and improved reliability. Integration with cloud services and artificial intelligence will likely drive new functionalities, enabling smarter and more efficient communication solutions. For developers and IT leaders, staying abreast of these changes will be crucial to leveraging SIP trunk’s full potential.

Let’s harness the power of SIP trunks together to propel your business forward in this digital era. Get in touch with Flowroute to learn more.