What Is SIP Trunking? How It Works, Benefits, & How To Get It

April 9, 2024 9 min read

Joe Manna

Joe Manna

What is SIP Trunking & SIP Trunks?

What Is SIP Trunking?

SIP (Session Initiation Protocol) trunking is a modern technology that enables voice and multimedia communications over an internet protocol (IP) network instead of traditional phone lines. It uses SIP, an application-layer protocol, to establish, modify, and terminate communication sessions between two or more endpoints.

The term “trunking” refers to the consolidation of multiple communication channels or lines into a single, logical connection or “trunk.” SIP trunks provision Voice over Internet Protocol (VoIP) connectivity between an on-premise phone system and the public switched telephone network, also known as PSTN. 

For instance, an office might have an existing PBX for phone service. SIP trunks provide phone service for the entire office so they can reach the outside world.

Running your small business phone system through SIP trunking is affordable, but it’s not necessarily for everyone. It replaces the need for physical lines. It also lets you easily add new phone lines or modify your phone service without disruptions or the need to buy new hardware.

If you’re still unsure about SIP trunking, Tom from Nextiva gives a great explanation in this 2-minute video:

SIP trunking can revolutionize your business by letting you upgrade your PBX to an internet-powered phone system.

It operates similarly to HTTP, which is the very basis of browsing web pages on the internet. SIP was embraced by the open-source community and acted as a catalyst for business phone service. Also, SIP’s capabilities extend beyond phone service into broader messaging functions.

Just like you’d get a traditional phone line subscription from a landline provider, SIP trunking is provided by a SIP provider.

Get your business phone system. Simple. Affordable.

Get your business phone, messages, video meetings, contact management and notes–integrated in ONE powerful app.

An excellent SIP trunking service lets you use your existing PBX equipment to transition into an internet-powered telephone network.

And that’s not all: you can also use a SIP phone as part of your unified communications system. You can synchronize all your communication channels in real time. This way, you’ll enhance productivity, collaboration, and efficiency across your company.

How Does SIP Trunking Work?

SIP trunking works as an intermediary between your business phone system and the Internet Telephony Service Provider (ITSP). See the diagram below showing how calls flow from a SIP phone (VoIP) in a business to the outside world — it’s fascinating!

SIP Trunking Diagram on how SIP trunking works in a VoIP Business Phone System

SIP trunking replaces traditional phone lines with a virtual connection, allowing your business to make and receive calls using your existing internet connection. It acts as an intermediary between your on-premise phone system (PBX) and the Internet Telephony Service Provider (ITSP) that delivers your service.

Think of it this way: traditionally, businesses relied on physical phone lines to connect calls. SIP trunking bypasses these lines by converting voice calls into digital data packets that travel over your internet network. This eliminates the need for expensive installations and clunky hardware.

Transitioning to SIP trunking

Here’s what you’ll typically need to make the switch to SIP trunking:

Direct inward dialing (DID) numbers: Enhancing flexibility

DID numbers are an additional feature you can leverage with SIP trunking. They provide a cost-effective way to assign specific phone numbers to employees or departments within your company.

SIP Trunking Diagram - SIP trunking connects existing communications hardware to the telephone network.
SIP trunking connects existing communications hardware to the telephone network.

Here’s how it works:

Using DID numbers with SIP trunking, you can enhance call management for various departments like customer service, sales, or even individual employees, improving overall communication efficiency.

What Is SIP Trunking Used For?

The main role of SIP trunking is to replace PRI technology. PRI, or Primary Rate Interface, has been used for decades to deliver lines of voice and data using physical copper lines.

Essentially, it’s a bundle of analog phone lines put together. PRI can be costly to maintain since the hardware is becoming outdated rather quickly.

With SIP trunking, this happens using VoIP technology. SIP also supports other data like instant messages and video conferencing.

SIP trunking is the right choice for anyone ready to move their business communication on a light budget.

This warrants a follow-up question: how will you know that your company is ready for SIP?
Here are some questions to consider:

Three steps to use SIP Trunking

In short, SIP trunking is used for call setup, management, and teardown. It’s flexible, reliable, and quite lightweight. This means you can focus on your business instead of the technicalities. SIP is the basis for VoIP communications, and SIP Trunking is used to provide VoIP connectivity through a PBX.

Benefits of SIP Trunking

Still unsure of the advantages of this approach to your business communications? In this section, we’re diving into the benefits of SIP.

SIP connects a variety of devices to connect individuals and businesses locally and globally through voice, video, and text. It simplifies business communication systems and makes businesses technologically competitive. Here’s how.

1) Cost savings and efficiency

With a traditional phone line, you may be paying for extra lines you aren’t using. You might also incur high charges for long-distance and international calls.

There’s also the cost of equipment maintenance.
With SIP trunking, you’ll experience simplified pricing, usually charged on a per-user basis. This comes with the extra benefit of predictability in your recurring expenses.

You get the choice between metered and unmetered SIP trunk plans. There’s a huge difference.

2) Virtual presence

SIP creates a unified virtual presence. It allows individuals and businesses to choose when and how they would like to be available.

For example, if you’re in a business meeting, calls and messages can be forwarded to your assistant. If you’re on a business trip, they’re routed to a smartphone.

Thanks to SIP, you’ll have centralized communication. We all have multiple ways we communicate: multiple devices at home, just as many (or more) at work, texts, video chat…The list goes on. SIP grants a user one “address” for all these devices and applications.

As a result, your interactions remain frictionless, no matter where you are.

3) Space for growth and scalability

When you’re looking to grow your company, your business needs will change. Sometimes, they’ll change at a rapid pace.

You need your technology to grow your business. With SIP trunking, you’ll combine your communications across channels and multiple offices into a single network.

Your call volume or the number of concurrent calls is likely to grow as your company grows. Technology shouldn’t stop you from doing things right.

No need for new IP infrastructure or waiting to speak to a customer service agent just to add new phone lines! With SIP trunking, you’ll be able to do all of that on your own, on demand.

4) Reliable technology

Traditional telephony service can be disrupted due to phone failures, bad weather, or a damaged telephone pole.

Luckily, SIP trunking with VoIP boasts higher reliability. This means that you can keep using your phone even in case of failure. Calls can be routed to other lines, offices, or your mobile device.

No matter what happens in the background, you can keep your business going smoothly and interruption-free.

Drawbacks of SIP trunking

While SIP trunking offers many advantages over traditional telephony services, it also has some potential drawbacks that businesses should be aware of:

🌐 Dependence on internet connectivity

SIP trunking relies on your internet connection to transmit voice data. If your internet goes down, your phone system will be inoperable. This can be a major issue for businesses that depend heavily on reliable phone communications.

How to overcome it: Implement redundant internet connections from different providers, or use a backup like 4G/5G cellular data to ensure continual internet connectivity.

🎛 Quality of Service (QoS) issues

Since voice data travels over the public internet, latency, jitter, and packet loss can degrade call quality if the internet connection is not properly optimized for real-time communications.

How to overcome it: Implement QoS prioritization policies on your network equipment to prioritize voice data. Use a dedicated internet connection for SIP trunking traffic.

VoIP QoS Illustrated Example
Routers prioritize voice traffic using QoS alongside other network demands.

🔐 Security vulnerabilities

SIP communications are transmitted as data packets, making them vulnerable to interception and hacking if not properly secured.

How to overcome it: Use encrypted SIP trunks, and enable security features like firewall protections, authentication, and data encryption. Implement robust cybersecurity practices.

🔌 Power dependency

SIP phone systems require power to operate, unlike traditional analog phones that derive power from the telephone line.

How to overcome it: Deploy backup battery/generator power for SIP phones. Consider keeping analog phone lines for emergency use.

Being aware of these potential drawbacks and implementing appropriate solutions helps businesses mitigate the risks and take full advantage of the benefits of SIP trunking.

PRI vs. SIP Trunking vs. VoIP

While PRI, SIP Trunking, and VoIP are related technologies used in voice communications, they differ in their capabilities and implementation. Here’s a side-by-side comparison:

FeaturePRISIP TrunkingVoIP
TechnologyTraditional phone linesInternet Protocol (IP)Internet Protocol (IP)
ScalabilityLimited, requires additional physical linesHighly scalable, no physical line limitationsHighly scalable, no physical line limitations
CostHigher costs for maintenance and scalingLower costs, no physical line expensesLower costs, no physical line expenses
MobilityLimited to physical locationsSupports remote workers and mobilitySupports remote workers and mobility
FeaturesBasic voice callingDepends on the specific protocol used (e.g., SIP)Advanced features like video conferencing, integration with business apps
FlexibilityRigid, aging technologyFlexible, adaptable to changing needsFlexible, adaptable to changing needs
Future-proofMay become obsolete as telecom providers phase out supportFuture-proof as it uses IP networksFuture-proof as it uses IP networks

For modern businesses, SIP trunking and VoIP solutions are generally more future-proof, cost-effective, and feature-rich compared to the aging PRI technology.

SIP Trunking vs. VoIP - Comparison

How To Get Started With a SIP Trunking Service

Getting started with a SIP trunking service doesn’t have to be complicated.

Who can benefit from SIP trunking?

SIP trunking is a valuable solution for businesses of all sizes, particularly those with:

Transitioning to a SIP trunking service is straightforward but requires some key steps. Here’s how to get started:

With our plans, we give you all of the above control and power over your cloud PBX — and more.
We also have an award-winning customer service team to help you with anything you may need along the way.

Sounds good?

Check out how you can power up your entire phone system with our cloud-based SIP trunks.

Joe Manna


Joe Manna

Joe Manna was a senior content marketing manager at Nextiva. He blends his marketing acumen and deep technical background to improve people's lives with technology. His expertise helps companies large and small serve more customers. He enjoys a rich iced latte and a non-fiction business book when he's not pressing words.

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