With traditional voice services such as standard landline systems coming to an end, many businesses will need to readjust their communication systems when the network ceases. The switch-off date for the UK’s PSTN (public switched telephone network) is expected in 2025, and, unfortunately, this isn’t a simple fix that will be resolved on its own.

At Excalibur, we are guiding more of our customers through the SIP trunking process as they upgrade their outdated telephone networks. In turn, they are experiencing a flexible communications system that is seamless with their business’s needs. We have detailed what SIP trunking is, its benefits and how you might expect it to work below.

What is SIP Trunking?

SIP trunking is a service that streamlines call delivery. A type of Voice over Internet Protocol (VoIP), Session Internet Protocol (SIP) simply refers to calls that are made and transmitted via a cloud-based server.

Similar to the anatomy of a tree, the term trunking is used to describe the collection of phone lines between a business’s private telephone exchange and the public switched telephone network. Essentially, SIP trunking compiles all phone lines as a connectivity base that allows you to make and receive calls without the use of a traditional telephone network.

SIP trunking works as a replacement to an ISDN (Integrated Services Digital Network) phone line. Instead of following a traditional telecoms route, SIP trunking redirects your business’s call traffic over an internet connection.

Where an ISDN system is in place, it utilises digitalised circuits from the public switched telephone network (PSTN) to direct voice traffic.

How Does SIP Trunking Work?

SIP trunking routes your existing private branch exchange (PBX) to an internet-based phone system. Once installed, any voice traffic will be carried out via an internet connection rather than through a traditional circuited telephone network.

Where a PBX or server is already in place, SIP trunking can be utilised. As well as offering flexibility with calls and allowing for a higher volume of call data to be processed, SIP trunking can also provide opportunities for analytics insight. Businesses can access call logs and measure someone’s availability and usage more accurately, in a GDPR compliant way. Statistics can be reviewed to provide feedback and evaluate strategies. It’s also possible to have single SIPs with multiple channels, meaning calls can be made simultaneously. Upon set-up, a channel limit will be determined.

SIP trunking has major benefits for businesses that communicate with overseas markets. Telephone calls cost significantly less this way, especially when calls to other geographical locations are a regular occurrence.

Under a traditional voice service, the telephone line connects with copper wires to establish calls. When phone lines have a greater distance between them a longer copper wire must be used, which ends up being quite costly. SIP trunking eradicates most of that cost by redirecting calls over the internet. It also allows for calls to be made via telephone, laptop or mobile devices which reduces the need to invest in regular maintenance and expensive hardware around the office and other global sites.

Difference Between SIP Trunking and PSTN

Although SIP trunking and PSTN are both used for voice data and communication, they can offer different capabilities depending on the services you require. With complete switch off for all ISDNs imminent, it’s worthwhile investigating their differences and how this could impact your business.


Whether a business is starting from scratch, expanding its team, or exploring new geographical markets, using SIP trunking instead of relying on traditional voice services could be a simpler option. With BT removing its PSTN and ISDN services by December 2025, it will  affect a lot of businesses that utilise voice services as well as alarms, door entry systems and even CCTV.

SIP trunking can be set-up virtually without the intrusion of engineers disrupting workflow. Businesses that look to scale up or venture into new territories will also have their worries removed when it comes to the administration of setting up new phone lines.

As experts in SIP trunking, at Excalibur, we are able to implement SIPs over existing connectivity, devoting time to communicate with your business and update you each step of the way.


For all PSTN systems, a dedicated line is required to start and complete any calls. Due to the nature of the system, it restricts the line to one call at a time, unless an ISDN line is in use. This can be quite limiting, and expensive, if hundreds of calls happen within your business each working day. Likewise, calls overseas or further away can be more costly when directed via an analogue telephone line.

When SIP trunking is introduced, each line can be used multiple times whilst other calls are ongoing, subject to channel limit at installation. Whilst line rental will still be an expense to consider, it could be up to 50% cheaper and significantly reduce call cost. In comparison to companies using the PSTN to conduct calls abroad, costs can also be somewhat decreased, dependent on current set-up and usage.


As businesses grow, their telecommunications system will need to be upscaled to keep up with demand. Scalability might be vital if pipeline value has significantly increased and a business’s services become highly appealing to their customer base. Utilising an ISDN system could considerably impact a business that is attempting to scale up.

When new talent is brought in as part of the process, adopting SIP trunking will save a significant amount of time when assigning telephone numbers. Additionally, if new geographical markets are ready to be explored, SIP trunking eradicates most of the time that would have otherwise been spent on creating the communications infrastructure for that business. With SIP, it’s possible to locate and buy local phone numbers within minutes, meaning quick and easy integration for new colleagues or offices. It’s important to note that porting existing numbers can take longer with additional charges required for custom or specific numbers.

Likewise, if a business needs to completely pare back their offering or adapt it for seasonal reasons or trends, SIP trunking allows for almost instantaneous reduction.


ISDN systems do not require the bandwidth capacity that VoIP technology does. As calls and voice data are directed over the internet, there is a greater reliance on bandwidth speed to cope with the higher volume of traffic. Enhancing the bandwidth capacity for SIP will also provide the possibility to conduct more calls overseas and can support extra office sites.


With traditional ISDN systems, maintenance and issue reporting is normally down to you to identify and react to. Maintenance can also be costly here, especially where phone line set-up needs to take place or be rerouted. SIP trunking requires very little physical infrastructure to set up. Where a server is not already in place, this will need to be added as it is vital component for SIP trunking.

What are the Benefits of SIP Trunking?

With the PSTN switch off taking place in 2025, SIP trunking is an ideal solution to future-proof your business. As services are due to shut down, any business that has not modernised its telephone infrastructure will be caught out. This could seriously impact business operations and prove to be business-critical in the long run. SIP trunking holds many benefits, from reliability to simplicity:

1. Efficient, reliable and flexible systems
2. Allows for futureproofing of business
3. Offers business continuity during emergencies or power failure
4. Available for remote and in-office workers
5. Involves simple network architecture
6. Minimal disruption during installation


Costs to Consider When Investing in SIP Trunking

• SIP offers cheaper day-to-day running costs than traditional voice services.
• Line rental could cost up to 50% less – if your business frequently utilises telephone communications, it could result in larger savings.
• If there is limited use of telephony within the business or a server has not been set up, this could be slightly more costly, but necessary for business continuity.
• Appropriate bandwidth capacity will be required to support higher volumes of traffic.
• Your company’s communication system might be outdated, or require updating, before SIP trunking can be implemented.

How Can SIP Trunking Improve Business Telephony?

Future Proofing Business 

Adopting a PBX or cloud-based server will enable a more seamless transition for your business telephony. As well as providing greater opportunities for scalability over the years, it will also be a good foundation when further challenges occur in the market, such as sustainability agendas and agreements.

Flexible Working Options 

Since COVID-19, flexible and remote working options have increased in popularity. If your business looks to endorse more opportunities for flexible working, SIP trunking is an ideal solution to achieve that. Offering hosted telephony options for alternative devices, such as smartphones or laptops, can enable employees to stay connected wherever they are based.

Business Continuity

When an emergency occurs, SIP trunking  offers business continuity. Calls can be quickly and seamlessly redirected to alternative locations, ensuring streamline resolutions. Potential disasters range from life-threatening weather conditions to infrastructure malfunction, and given that they are impossible to predict, it’s important to have a contingency plan in place.

System Integration 

SIP trunking can also allow for integration of voice, video and other dynamic forms of interactive conferencing across your communications tools. Some VoIP providers can facilitate the communication between vital programmes that your business uses.

Additional Features for Analytics 

Encrypted data can be stored from call traffic and analysed. This insight could offer opportunities to review effectiveness of marketing campaigns. It can also allow for simple tools, such as viewing an employee’s working status and call recording.

Are you looking for set-up support with SIP trunking?

If you are looking to get ahead of the BT, Virgin Media and KCOM PSTN switch-off, then, at Excalibur, we can help your company start the SIP trunking process. If you need advice about the benefits of SIP or how to switch to it, our consultants are available to give you advice.  

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