What is SIP trunking, how does it work, and what other options does your business have?

With more and more businesses making the move to internet telephony, you don’t want to get left behind. Especially as telecom providers worldwide are gradually switching from physical copper-based landlines to internet-based phone services.

The thing is, if you have an on-premises PBX system with analog phone lines, you might not want to part with it just yet. That’s where SIP trunking comes in, helping you upgrade your comms without ditching all your legacy equipment.

But what is SIP trunking, precisely? How does it benefit your business? And, most importantly, why might one of the alternative solutions work better for you?

SIP trunking: What is a SIP trunk & what is it for?

We’ll get started with a SIP trunk definition. SIP trunking is a technology that allows businesses to place digital phone calls using an existing on-premises PBX.

A Private Branch Exchange (PBX), if you didn’t know, is a telephone network based and used within an organization. It connects plain old telephone service (POTS) lines to a public switched telephone network (PSTN).

SIP stands for Session Initiation Protocol, which is a signaling protocol (a set of rules) for initiating, maintaining, and terminating communication sessions between two or more endpoints over an IP (Internet Protocol) network.

A SIP trunk is a set of virtual phone lines or communications channels, consolidated into a single connection or “trunk.” This may consist of 20 or more SIP lines or channels. 

What are SIP trunks used for? Well, they allow organizations to make and receive phone calls over the internet, instead of using traditional landlines.

SIP trunking facilitates VoIP (Voice over Internet Protocol) connectivity between your on-premises phone system and the PSTN. This means you don’t need physical trunk lines connecting the two, nor an ISDN (Integrated Services Digital Network) circuit at your office.

In short, SIP trunking is a way of upgrading your PBX to a digital phone system, from which you can make both PSTN and VoIP-based calls.

SIP trunking vs PRI

Before SIP came along, the connection between legacy PBXs and the PSTN took place over phone lines called primary rate interfaces (PRIs). PRI is a type of digital technology, but it uses the ISDN (Integrated Services Digital Network) protocol to provide connectivity.

Each PRI consists of two pairs of copper wires, connecting the network to the end-user. Although it’s actually a single line, it can transmit up to 23 calls simultaneously. Any more than that, and you’d need multiple PRIs.

PRI requires trained technicians for installation and maintenance, so it can be pricey—and the hardware is fast becoming obsolete. It’s inflexible, not very scalable, and vulnerable to local disruptions. The other downside of PRIs is that they can only carry voice traffic—whereas SIP trunks support multiple forms of data traffic.

How does SIP trunking work?

Now that we understand the SIP trunk meaning, let’s take a closer look at how it all works.

Your existing PBX provides internal communication between your various office locations. If you want to make or receive external calls, you have to be connected to the PSTN. SIP trunking is a more modern way of achieving that connection. 

It acts as a middleman between your business phone system and the Internet Telephony Service Provider (ITSP) so that you can make and receive calls via your existing internet connection. 

When someone calls your business, the SIP trunk directs the call through the internet to your phone system. When you dial an external number, the call is routed via the SIP trunk to traditional phone networks. In both cases, voice signals are converted into digital data “packets” to travel over the internet network.

There are two types of PBX systems: analog or legacy PBX (which only has phone lines for internal calling), and IP PBX (which already supports the use of SIP). If you have a legacy PBX, you’ll need a gadget called a Session Border Controller (SBC) to connect the SIP trunk to the phone system, plus Analog Telephone Adapters (ATAs).

If you’re setting up a SIP trunk, you’ll need to make sure that you have enough internet bandwidth to accommodate VoIP calling. It’s also recommended to configure your router and firewalls to allow SIP traffic, by implementing a dedicated voice VLAN or QoS rules to prioritize voice packets.

Benefits of SIP trunking

In comparison with traditional on-premises systems—connected the old-fashioned way to the PSTN—there are many advantages of SIP trunking.

Cost-savings and protecting existing investment

Using SIP trunking helps you to reduce communication costs, especially if your business makes a lot of calls. That’s partly because SIP trunking services typically come with simplified pricing, charged per user or per minute.

You can usually choose between metered and unmetered SIP trunk plans, with shorter contract durations than you’d get with a traditional phone line. Landlines also charge higher prices for long-distance and international calls.

Crucially, SIP trunking enables you to keep using your existing on-premises PBX. It’s likely that you invested heavily in setting this up, not to mention what you’ve spent on maintenance over the years. Instead of scrapping the equipment, you can use it as the basis of a new digital network.

More communications options

A traditional on-premises PBX, connected to the PSTN via physical phone lines, doesn’t give you many options for modern communication. SIP trunking can support instant messages and video conferencing alongside voice calls.

And you’ll have the ability to customize call management settings such as routing. For example, you might choose to have calls and messages forwarded to a team member or even routed to your smartphone when you’re away from the office.

With SIP trunking, call quality improves and you can encourage your customers to get in touch via virtual local and toll-free numbers.

Greater scalability

Another key advantage of SIP trunking over legacy alternatives is that it can make your phone system much more scalable. Let’s say you want to add new phone lines. With a traditional system, reliant on PRI, the copper wire lines would have to be installed physically—but SIP lines are virtual.

SIP trunk services are easier to manage than legacy phone systems. You can add or remove lines whenever you need to, catering for fluctuating call volumes. The ability to handle multiple calls simultaneously is also good for growing companies.

Limitations and drawbacks of SIP trunking

An on-premises PBX connected to the PSTN via SIP trunking isn’t always the best solution. Here are some SIP trunk drawbacks that might sway you toward an alternative:

Quality of Service (QoS) issues

If it seems like traditional landlines are getting a bad rap in this post, they do have their upsides. Call quality is one of these, since analog phones are extremely reliable and keep working even in a power outage.

With SIP trunking, however, data travels over the public internet. If your internet connection isn’t properly optimized, latency, jitter, and packet loss can affect call quality. This may also happen if the SIP trunk is split into too many channels.

You can mitigate call quality problems by using the QoS settings on your router to prioritize SIP traffic for audio over less-demanding data traffic. 

Security concerns

Again, landline-based systems are reliable in terms of security—it’s very hard to hack into those systems, with the exception of wire-tapping. But any form of digital technology (such as SIP trunking) is potentially vulnerable to cyber-attacks.

Because SIP communications are transmitted over the public internet as data packets, it’s possible that they could be intercepted and hacked. To guard against things like Denial of Service (DoS) attacks, spoofing, eavesdropping, and fraud, you’ll need to check that your SIP trunking provider is hot on security.

Hardware maintenance

Another of the drawbacks to SIP trunking is that although you have more flexibility and scalability, you still have your on-premises PBX to maintain. That’s usually a task for IT professionals, which incurs a cost. And as the hardware gets older, the task becomes harder.

You may need to update your PBX system in preparation for SIP trunking, and you’ll have to configure the new service with your Internet Telephony Service Provider (ITSP). Setup also involves checking the compatibility of things like firewalls and routers, and you may need gateways, or adapters to connect with other platforms and devices. 

You could instead opt for a hosted PBX or full-fledged cloud phone system where there’s no hardware to install or maintain.

More limited feature set than cloud-based VoIP phone systems

As we’ve discussed, SIP trunking gives you access to more functionality than a traditional landline-based system. But that still may not be enough for a growing business in the age of digital and omnichannel communications. A cloud-based business phone system will include more comprehensive features than those you can achieve with SIP trunking.

Cloud technology also makes it easy to add or remove features on demand, and connect to your organization’s CRM, apps, and other software. It integrates multiple communication tools into a single, streamlined platform, instead of using separate apps.

Implementing SIP trunking: Why do businesses turn to SIP trunk providers?

If you want to modernize your business phone system, you may choose to go down the SIP trunking route. This is an option for growing companies experiencing increasing call volumes or needing to make international calls. You might simply want a more flexible and scalable solution.

In this instance, you can turn to a SIP trunk provider to help you digitize the connection between your on-premises PBX and the PSTN. SIP trunking providers are the entities who supply the virtual phone lines and the phone numbers that go with them.

As part of your purchased SIP trunk service, most providers will assist with installation, onboarding, and porting existing numbers to the new service. This should minimize any interruption to your business. 

Businesses also rely on SIP trunking providers to:

• Ensure failover to backup trunks in the event of an outage
• Provide security, such as encrypted voice calls and flagging anything suspicious
• Route calls to emergency services with caller location information
• Offer virtual phone numbers, including local and toll-free numbers
• Route calls to multiple locations according to your rules
• Add new lines to the SIP trunk to allow communications between your network and other phone networks, such as cellular
• Generate reports on usage, availability, and performance.

SIP trunking vs. VoIP phone systems and UCaaS

SIP trunking is just one way to enable digital communications for your phone system. You can also do this with a VoIP phone system or a Unified Communications as a Service (UCaaS) solution.

Whether you’re thinking of implementing SIP trunking for the first time or planning to move your on-premises PBX to the cloud, here’s a direct comparison of three types of communications systems:

As we know, SIP trunking leverages your on-premises PBX system to provide VoIP (and PSTN) calling. VoIP and UCaaS solutions are hosted remotely in the cloud by a vendor, who provides the service, owns the infrastructure, and handles all maintenance. This is also sometimes known as hosted PBX.

By eliminating on-site hardware, you’re automatically cutting the cost of installation, setup, and maintenance—and you’ll have more scope for expansion. SIP lines are virtual, but a system that’s based entirely in the cloud gives you more flexibility to offer remote working.

Although the costs of a cloud-based phone system vary depending on the provider and the type of plan you choose, the subscription payment system helps you with budgeting. It’s similar to having a pay-as-you-go plan with your SIP trunking provider, but the difference is that it’s easier to make adjustments with a cloud service.

SIP supports multimedia communication and call management, but it tends to have fewer built-in features than VoIP or UCaaS. And UCaaS has the major advantage of unifying all your comms in one platform, rather than using different apps for each one.

SIP trunking or a cloud-based VoIP phone system?

It’s a big decision. Ultimately, your choice will probably depend on whether or not you already have an on-site PBX in place. If you do, then we understand why you might be unwilling to get rid of it. You paid a lot for installation and setup, after all—plus ongoing maintenance.

As long as your PBX system is still functioning properly, you may choose to add SIP trunking in order to expand the features and scalability. (Remember that a legacy PBX will be a little harder to configure than an IP PBX.)

You might already have some comms tools, such as team chat, in a separate system, and want to use SIP trunking to add VoIP telephony. Another reason for going down the SIP trunk route is that you want to retain control over your business phone system. Maybe you’re concerned that your employees will be resistant to change. Don’t forget, though, that change may ultimately be inevitable—plenty of nations are phasing out the PSTN entirely.

If you don’t already have a PBX—or if you’ve decided that the time for true modernization is now—your best bet is to embrace the cloud. Hardware-free and easy to manage, VoIP phone systems and UCaaS solutions give you affordable calling and a full set of features.

RingCentral RingEX: A more future-proof choice than SIP trunking

Overall, a cloud-based phone system is more flexible and cost-effective than SIP trunking. And if you really want to ensure that your business communications are fit for the future, a UCaaS platform is the way to go. Specifically, in our humble opinion, RingCentral RingEX.

With RingEX, you get voice calls, video meetings, messaging, SMS, and fax, as well as file-sharing. Users enjoy HD call and video quality with noise cancelation, plus unlimited domestic calling in the USA and Canada, and affordable international rates.

RingEX comes with a raft of AI-powered features and more than 400 integrations with other business tools. The built-in AI Assistant boosts efficiency by providing live call notes, personalized insights, and message writing and translation.

You’re in control of users, permissions, and call flows across multiple locations. You can customize routing and add new users and features—no IT expertise required. Since everything happens in the cloud, you won’t have to worry about hardware or maintenance.

With 99.999% uptime and world-class security, the AI-powered unified communications from RingCentral takes the digital experience beyond anything SIP trunking could offer.

SIP trunking FAQs

What is a SIP trunk line?

A SIP trunk line is a group of virtual phone lines, gathered into a single connection called a trunk. It is a way of digitally connecting an on-site PBX to the PSTN (the main landline telephone network), enabling calls (and other communications) to take place via the internet instead of copper wires.

What’s the difference between SIP trunking and VoIP?

SIP (Session Initiation Protocol) is a signaling protocol that enables VoIP calls. VoIP (Voice over Internet Protocol) is a set of voice technologies that support various internet protocols. While VoIP transmits only voice data (unless it’s part of a business phone system with extra features), SIP trunking supports multimedia communications.

Why do some businesses choose SIP trunking?

A business that has invested in an on-premises PBX system might choose to digitize it with SIP trunking, instead of scrapping it entirely. SIP trunking facilitates cheaper calling than using traditional landlines, and adds flexibility and scalability to the existing system.

What are the benefits of a cloud-based VoIP communications platform over SIP trunking?

SIP trunking has limitations in terms of features, security, and call quality—and you still have to carry out increasingly costly maintenance on your PBX hardware. With a cloud-based VoIP comms platform (or a UCaaS solution), the provider owns and manages the infrastructure.

You typically get unlimited domestic calling, extra features and integrations, and top-notch security. And all your communications will be streamlined in one place.

Originally published Dec 10, 2024