What is SIP Trunking: A Guide for UK Small Businesses

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Understanding SIP trunking is beneficial if your company is transitioning to VoIP phone services. Despite the somewhat confusing name, it doesn’t take long to grasp the fundamentals of this powerful telecom technology and how to set it up for your business.

We’ve discussed how SIP trunking came to be, how SIP trunks work, and everything else you need to know below.

What is SIP Trunking?

SIP stands for Session Initiation Protocol. Trunking is a digital method of making and receiving phone calls and other communications over an internet connection that makes use of SIP, an application layer protocol. In this sense, “trunking” refers to the process of combining several communication channels into a single link, allowing for more efficient resource use and connectivity to a telephone network.

SIP trunks connect an on-premise phone system to the public switched telephone network (PSTN) using Voice over Internet Protocol (VoIP). For example, an office may already have a PBX for phone service. SIP trunks provide phone service for the entire office, allowing it to communicate with the outside world.

Running your small business phone system using SIP trunking is inexpensive, but it is not for everyone. Physical lines are no longer required. It also allows you to simply add extra phone lines or change your phone provider without causing disruptions or purchasing new hardware.

How Does SIP Trunking Work?

Previously, businesses used ISDN circuits, or copper lines, which they physically installed on their premises. SIP trunking replaces analogue phone lines with digital phone lines. The method works by dividing calls into ‘digital packets’ and sending them over a data network. The various components of the SIP trunking system are explained below.

#1. SIP channels

SIP channels are the digital equivalent of a phone line. Each SIP channel supports two simultaneous calls, one outbound and one inbound. Unlike physical phone lines, SIP channels can be added as needed without the need for wiring.

#2. SIP trunks

SIP trunks are groups of multiple SIP lines, often more than 20. The SIP trunk serves as a conduit between a company’s phone lines and its Internet Telephony Service Provider (ITSP). Consider the SIP trunk to be a virtual phone line connecting a home or business to a telephony service provider. It enables communication between the corporate network and multiple other phone and data networks, both inbound and outbound. You can connect your PBX, for example, to the Public Switched Telephone Network (PSTN).

#3. SIP protocol

The Session Initiation Protocol (SIP) is a set of communication rules that govern the establishment and termination of data connections over the Internet. It enables an online voice communication session so that users can communicate with one another via phone, video call, or messaging service.

#4. SIP provider

SIP providers are firms that provide SIP services such as voice calls, video conferencing, and instant messaging. A SIP provider aggregates one or multiple SIP connections and runs them through a company’s on-premise private branch exchange (PBX). Those SIP lines are then run through an SIP trunk, with calls and other digital communications enabled over an internet connection. The choice of SIP provider depends on the services your organisation wants.

All SIP carriers offer SIP trunking to organisations with a PBX, but only a few provide an Internet Protocol PBX (IP PBX) for firms that do not have an established PBX system. This means organisations that have nothing but phone connections can start adopting SIP trunking.

Who Should Consider Them?

In short, any business! If you already have an IP PBX, you’ll know (or should know) that SIP trunks are an absolutely necessary component. But if you’re new to phone systems or are migrating from an analogue PBX, you have a choice to make between an on-premise (commonly shortened to “on-prem” in industry lingo) IP PBX or a fully cloud-hosted PBX system.

Using on-premise hardware combined with SIP trunks is particularly suitable to enterprises with these characteristics:

#1. IT managers and telephone professionals in-house

On-premise PBX systems need professionals to set up and maintain them. These can be agencies and managed service providers, but they will also need at least one in-house contact with some knowledge of telephony. In the near term, on-premise PBX systems can be highly expensive when combined with relevant setup and maintenance.

#2. A High Call Volume

Any company or location that receives a high volume of outbound and inbound calls requires the flexibility and cost-effectiveness that SIP Trunking delivers. Call centres, such as sales or support centres, are more likely to use more complex PBX systems supported by high-volume SIP services.

Choosing the correct SIP provider for their calling profile might imply significant cost savings in the long run, with the drawback of paying more on gear and setup in the short term.

For other sorts of businesses, hosted PBXs may be a preferable option because they provide a comparable range of capabilities and reliability while incurring nearly none of the maintenance and setup costs associated with on-premise options. A good VoIP provider should always assist you in selecting the best option for your business.

What are the Benefits of SIP Trunking?

SIP trunking is appropriate for enterprises that require several phone lines, ranging from sole proprietors and small businesses to large corporations with many global locations. If you make a lot of calls or believe your call volume will increase in the future, SIP trunking could be beneficial to your company.

#1. Financial savings

Traditional phone lines can be expensive, especially if your organisation has numerous unused lines or makes frequent international calls. Businesses must also cover the price of hardware and maintenance. SIP trunking is typically charged per channel, for example, based on the maximum number of concurrent calls you anticipate making. Some services allow flexible pricing, so you pay just for what you use (unmetered vs. metered).

#2. Scalability

If you have multiple locations and use the same carrier, calls between sites are usually free (you only pay monthly rates for the data circuits like broadband). You can scale your internet connection to suit your intended maximum call volume. The required bandwidth is determined by the type of media (voice, video, messaging) and the codec used. Because fibre broadband has very high bandwidth rates, it can support a large number of concurrent calls.

#3. Dependability

SIP trunks have the potential to be more reliable than traditional analogue circuits. To begin, number failover is possible, so that if the primary number cannot be reached, it can failover to a secondary number, possibly in a different location. Second, unlike traditional phone lines, SIP trunks do not have a defined route end-to-end for routing calls.

#4. Analytics and insight in real time

Because SIP trunking systems are cloud-based, you may get analytics and data insights in real-time. You may easily access call logs to accurately monitor usage and availability. You can use machine learning services to extract business intelligence from the data. Statistics can be split down to individual users, giving you a glimpse of each employee’s interactions with customers or prospects.

This real-time data access assists your company in improving staff performance and driving gains in marketing, customer service, and sales.

#5. Centrally managed communication

Teams can use SIP trunking to make and receive phone calls, video conferencing, and messaging from a single place. Previously, these services would have been dispersed over various locations, making communication inefficient and costly. Once configured, a SIP trunking communications system is simple to manage. All of your company’s digital communications will be integrated into one platform by unifying your communications system.

#6. Call quality

SIP trunks are intended to provide enough bandwidth for your digital phone connections to make high-quality calls. You can expect clear calls even during busy periods if your private branch exchange (PBX) is not overburdened. With advances in artificial intelligence, we now have technology that automatically improves call quality and fixes errors like echoes and static. You shouldn’t have to worry about call drop-outs or echoey connections.

#7. Security

Unlike earlier phone systems, your SIP trunks are dedicated connections for your calls. SIP providers monitor calls for unusual conduct as part of their service, meaning anything unexpected will be noticed quickly. It is worthwhile to inquire about your SIP provider’s individual security protocols.

What Do I Need to Consider When Making Research?

When researching SIP Trunk services, keep the following considerations in mind:

#1. How many simultaneous calls will you require?

Remember that the number of channels required is determined by the number of concurrent calls, which includes both inbound and outgoing calls. It’s beneficial to have a basic awareness of your company’s concurrency, but don’t worry if this is difficult for you. One significant advantage of SIP services is their flexibility; adding and removing channels is simple.

#2. How much time would you require?

A simple question, but how many phone calls do you actually make? Many carriers may bundle minutes and channels, but for some, making calls based on a defined rate may be cheaper. This is typically only true for very low-volume call businesses.

#3. How many new phone numbers would you require?

Why not obtain some fresh numbers if you’re looking for a new provider? This is especially important if you’re launching a new office in a different geographic area (perhaps a different county or even nation). SIP allows you to access the full benefits of VoIP services. This can make small businesses appear much larger and more efficient than they are.

#4. Are you transferring any existing phone numbers?

Unless you’re a brand-new firm, you’ll most likely already have a business number. You don’t want to lose touch with consumers who are used to calling a specific number for sales or support, therefore porting becomes critical. Porting is straightforward but does take slightly longer than other components of the SIP Trunk process, often lasting approximately 1-2 weeks.

SIP Trunks vs SIP Channels

“Trunks” are typical sets of cables that connect the local phone exchange to the exchange. This technique became virtual with the advent of the internet. A trunk is now a collection of designated “channels,” each of which may handle a predetermined number of concurrent (happening at the same time) calls. The number of channels you’ll require as a business is proportional to the amount of calls you’ll be making and receiving at the same time. A decent SIP supplier will assist you in calculating this.

Channels are an important part of knowing how trunks are packed and delivered. Every SIP Trunk has a number of channels, which are essentially virtual equivalents of traditional phone lines. Each channel supports two simultaneous calls, one incoming and one outgoing. This is critical. If your company makes a lot of calls but rarely has more than two calls going at the same time, you won’t need more than one or two channels. If your company receives a significant number of inbound calls at the same time, you’ll need to consider a large number of channels.

What is the Difference Between SIP and VoIP?

SIP and VoIP are similar but not the same technology. Voice over Internet Protocol (VoIP) refers to all voice calls that are made utilising an internet connection rather than analogue phone lines. It is a catch-all name for the technology that enables internet voice calls. While SIP uses VoIP technology for its voice conversations, it can also offer multimedia services (video, conference calls, and messaging) across its protocol. SIP is a broader technology than VoIP.

What is the Difference Between SIP Trunking and PRI Trunking?

Instead of the virtual connection provided by SIP trunking, a Primary Rate Interface (PRI) system, often known as a PRI trunk, provides a physical connection to the PSTN. PRI employs copper cable in a circuit-switched paradigm to produce up to 23 voice channels.

What is a High-volume SIP Trunk?

A high-volume SIP trunk operates on a separate dedicated platform designed for high-traffic volumes. It allows you to utilise as many volumes as you like without any problems.

What is a SIP Channel?

A SIP channel can support both incoming and outgoing calls. Every SIP trunk includes a number of channels, which are virtual telephone lines. The number of channels required is determined by the number of concurrent calls made by your company.

What’s the Difference Between SIP and VoIP?

VoIP, or Voice Over Internet Protocol, is a technology that allows you to make and receive phone conversations over the Internet or internal networks. SIP is an application layer protocol for creating, managing, and terminating multimedia sessions such as phone, video, and message. While SIP is an initiative in and of itself, it is mostly used to facilitate IP technology, such as VoIP communications.

What Requirements Do I Need for a SIP Trunk?

The most basic prerequisite for SIP trunks is a high-bandwidth, high-speed internet connection. This is determined by the number of phone calls you anticipate making as well as other web applications. You will also need an IP PBX that supports the SIP protocol. If you do not have this, you may need to install an SIP card or a licence.


As more enterprises begin to migrate away from SIP trunking and towards the cloud, it is critical to plan for a smooth software conversion. Before switching from SIP trunking to a new business communications solution, evaluate your specific company requirements. Consider the elements that will assist you in creating a more flexible and scalable working environment.


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