For decades, PRI (ISDN30e) has been the backbone of business telephony in the UK, offering reliable fixed-line connectivity for voice services. But with BT set to retire the ISDN network entirely by 2027, organisations now face a critical turning point: whether to continue investing in outdated infrastructure or move towards the flexibility of SIP trunking.
This isn’t just a question of SIP trunking vs PRI on technical grounds. The reality is that PRI will soon no longer be available, meaning every business that still relies on ISDN must plan a PRI to SIP migration or risk losing vital communications capability. For many, this shift isn’t about if they will move but how and when they will do it. – See more about the switch off on BT’s website.
Fortunately, SIP trunks provide a natural ISDN30 replacement in the UK, combining lower costs with greater scalability, resilience, and future-proofing. As more organisations embrace cloud and hosted VoIP solutions, SIP has become the obvious path forward—ensuring that business communication remains agile, reliable, and ready for what comes next.
What Is PRI (ISDN30e)?
Before the rise of internet-based telephony, many UK businesses relied on ISDN30 PRI (Primary Rate Interface) lines to power their phone systems. A PRI circuit delivered up to 30 individual channels over a single digital line, making it a robust and scalable option for companies with heavy call volumes or on-site PBX systems. For years, PRI lines in the UK set the standard for enterprise-grade communications, offering predictable performance, high reliability, and consistent call quality.
In its time, ISDN30e was seen as cutting-edge, enabling larger organisations to consolidate their voice traffic while maintaining control over their phone networks. However, technology has moved on, and so has the way businesses communicate. With cloud platforms, remote working, and mobile-first strategies becoming the norm, ISDN-based services now look increasingly outdated.
That’s why BT has confirmed that all ISDN and PSTN services will be switched off in January 2027, leaving PRI customers with no choice but to migrate. The good news is that modern solutions like SIP trunking offer a direct ISDN30 replacement in the UK, providing more flexibility, cost savings, and future-ready connectivity.
What Is SIP Trunking?
SIP trunks are the modern digital equivalent of traditional PRI lines, but instead of relying on physical ISDN circuits, they use the internet to deliver voice connectivity. In practical terms, SIP (Session Initiation Protocol) allows businesses to make and receive calls over their broadband or leased line connection, providing the same core functionality as ISDN—only with far more flexibility.
One of the key advantages is compatibility. Businesses that have invested heavily in on-site PBX systems don’t need to rip and replace. With a SIP-enabled PBX or a suitable gateway, existing phone systems can be connected directly to SIP trunks, making the shift from PRI to SIP migration smooth and cost-effective.
Scalability is another major benefit. While PRI lines in the UK were fixed in 30-channel blocks, SIP trunks can be scaled up or down instantly to match business needs—whether you need five channels or fifty. This flexibility makes it easier to support seasonal demand, business growth, or even a move to hybrid working without the expense and delay of ordering new circuits.
Crucially, SIP trunks are often significantly cheaper than maintaining legacy ISDN services. With lower call charges, reduced line rental, and simplified infrastructure, many organisations find SIP a natural step towards modernisation. For companies weighing up SIP trunking vs PRI, SIP delivers a future-proof, cost-efficient, and resilient ISDN30 replacement in the UK.
SIP vs PRI: Key Differences
When weighing up SIP vs PRI, it helps to see the differences side by side. Both technologies were designed to deliver reliable business voice services, but their underlying infrastructure and capabilities couldn’t be more different. PRI (ISDN30) was built for an era of fixed office lines, while SIP trunking benefits from the flexibility of internet-based delivery.
Below is a simple comparison of the two approaches:
| Feature | PRI (ISDN30) | SIP Trunking |
|---|---|---|
| Infrastructure | Runs over physical copper ISDN circuits, tied to a single location. | Delivered over the internet or dedicated IP connections; not bound to one site. |
| Scalability | Fixed in 30-channel blocks; growth is inflexible and costly. | Add or remove channels instantly to match demand. |
| Costs | Line rental and on-site hardware; typically higher call charges. | IP-based pricing with lower setup, rental, and call costs. |
| Reliability | Dependent on copper; single faults can impact an entire site. | Supports redundancy/failover across multiple links and locations. |
| Flexibility | Restricted to the premises where lines are installed. | Works from any location with a reliable internet connection. |
This table highlights why so many organisations are already planning their PRI to SIP migration. SIP not only removes the limitations of physical infrastructure but also introduces cost savings, agility, and future-proof resilience.
In short, SIP trunking benefits businesses by delivering scalable, flexible, and cost-effective communication compared to the rigid structure of PRI. So on paper, SIP is the clear successor to PRI — but the market is moving even further.
The Limitations of SIP Trunks
While SIP trunks are often positioned as the natural upgrade path from ISDN, they are not without limitations. At their core, SIP trunks still connect into an on-premises PBX, meaning your business remains tied to local hardware. This setup can be perfectly adequate in the short term, but it does come with ongoing responsibilities.
Maintaining a PBX requires regular upgrades, patches, and technical expertise. If the hardware fails or your office suffers an outage, resilience is limited to whatever redundancy you’ve built into your own infrastructure. Unlike cloud-based services, SIP trunks don’t automatically provide geo-redundancy or disaster recovery — those protections are only as strong as the systems you put in place locally.
For many organisations, SIP is a good transitional step in a PRI to SIP migration strategy, extending the life of existing PBX investments while reducing the costs associated with ISDN. However, it’s not truly “future-proof.” Businesses still shoulder the burden of hardware maintenance, and scaling across multiple sites or remote workers can quickly become complex and expensive.
This is why an increasing number of companies are choosing to bypass SIP trunks altogether and move straight to hosted VoIP. With no on-premises equipment to maintain, hosted services offer greater flexibility, built-in resilience, and a communications platform designed for the cloud era.
Hosted VoIP: The Future-Proof Option
As businesses plan how to migrate from ISDN to VoIP, one solution is standing out as the clear long-term winner: Hosted VoIP. Also known as a cloud PBX or hosted PBX, this model removes the need for on-premises telephone hardware altogether. Instead, all call handling, routing, and management are delivered via secure data centres in the cloud, with users connecting through desk phones, mobiles, or softphone apps.
The benefits are immediate. With hosted VoIP in the UK, there’s no PBX to maintain, no hardware upgrades to schedule, and no risk of local equipment failure disrupting your business. For companies embracing hybrid or remote working, a hosted system is particularly compelling: employees can make and receive calls anywhere with an internet connection, while managers retain centralised visibility and control.
Hosted solutions also integrate seamlessly with modern business tools. From CRM platforms to Microsoft Teams and collaboration apps, cloud PBX UK services ensure telephony is no longer an isolated system but part of a unified communication strategy. Scalability is effortless too. Instead of adding physical channels, you simply add or remove users as needed — an ideal fit for growing teams, seasonal demand, or distributed offices.
Cost is another driver. Without the burden of on-site PBX hardware and its associated upkeep, many organisations find hosted VoIP delivers a lower total cost of ownership over time. You pay only for what you need, with predictable subscription pricing and no surprise maintenance bills.
When comparing hosted VoIP vs SIP trunks, the difference becomes clear. SIP trunks are an evolutionary step — extending the life of legacy PBXs and easing the transition away from ISDN. Hosted VoIP, however, is revolutionary. It eliminates the PBX entirely, replacing outdated infrastructure with a cloud-native model built for agility, resilience, and long-term scalability.
With the UK rapidly moving to an all-IP future, hosted VoIP isn’t just another option on the table — it’s the natural end-point for business communications. For organisations still weighing up whether to move from PRI to SIP migration or go further, the message is simple: SIP can buy you time, but hosted VoIP is where the journey ultimately leads.
Which Should You Choose?
With the UK’s ISDN and PSTN switch-off looming in 2027, the real decision for businesses is no longer PRI vs SIP, but SIP trunks vs Hosted VoIP. Each has its place, and the right choice depends on your current infrastructure and long-term goals.
If your organisation has recently invested in a modern PBX system, SIP trunks may provide the best short- to medium-term option. They allow you to sweat that asset for a few more years while still gaining many of the cost and scalability advantages of IP-based communications. For some businesses, this is a sensible way to protect past investment while bridging the gap from legacy technology.
However, if your priority is to future-proof communications, cut costs, and support hybrid or remote teams, then Hosted VoIP is the smarter long-term move. By removing the PBX entirely, you gain a system that scales instantly, integrates with cloud apps, and delivers resilience without on-site maintenance. It’s a platform designed for today’s all-IP, mobile-first workplace.
What’s certain is that PRI is disappearing. Waiting until the final months before the 2027 deadline risks rushed decisions, limited availability of migration services, and potential downtime. The safest strategy is to start evaluating your options now, map out your migration path, and make the transition in good time.
Whether you take an interim step with SIP trunks or move directly to a hosted solution, one thing is clear: the sooner you act, the better positioned your business will be.
Migrate From PRI Today
The message for UK businesses is clear: PRI is reaching its end of life. By January 2027, ISDN30 and all legacy PSTN services will be switched off, and organisations that haven’t migrated will be left without a working phone system. The question is no longer if you should move, but how.
For some, SIP trunks will provide a practical bridge — especially if you’ve recently invested in a PBX and want to extend its lifespan for a few more years. But ultimately, the market is moving toward Hosted VoIP, the cloud-first solution that removes on-site hardware, reduces costs, and supports the modern way we all work: mobile, remote, and connected.
The smartest move right now is to audit your current setup, explore the options for SIP or hosted VoIP, and plan your migration well before the 2027 deadline. Acting early avoids last-minute disruption and ensures your business communications are ready for the all-IP future.
Talk to Plexatalk about future-proofing your phone system today.
FAQs About ISDN30 / PRI in the UK – SIP Trunking vs PRI
What is ISDN30 (PRI)?
ISDN30, also known as PRI (Primary Rate Interface), is a digital telephony service that provides up to 30 voice channels over a single circuit. It has traditionally been used by larger businesses to connect PBX phone systems.
When will ISDN30 be switched off in the UK?
BT will retire all ISDN and PSTN services — including ISDN30 — in January 2027. After this date, PRI lines will no longer work.
Why is ISDN30 being phased out?
ISDN runs on copper networks that are costly to maintain and not suited to modern digital communication. The UK is moving to an all-IP infrastructure, making ISDN services obsolete.
What is the replacement for ISDN30 in the UK?
Businesses must migrate to IP-based solutions. The two main options are SIP trunks, which connect existing PBXs to the internet, and Hosted VoIP (cloud PBX), which removes on-premises hardware entirely.
Can I keep my numbers when moving from ISDN30?
Yes. During a PRI to SIP migration or a move to hosted VoIP, you can usually port your existing business numbers to the new system.
Should I choose SIP trunks or Hosted VoIP after ISDN30?
If you have a recent PBX investment, SIP trunks may make sense as a short-term step. For long-term cost savings, flexibility, and future-proofing, Hosted VoIP is the better choice.