By the end of 2025, the UK’s legacy telephone systems — including ISDN and PSTN — will be completely decommissioned.
Often referred to as the "BT ISDN Switch-Off," this transition marks a major milestone in the UK’s shift to modern, digital communication networks. If your business still uses traditional landline systems, it’s time to act. After 2025, ISDN lines will no longer function, meaning phone systems, fax machines, alarms, and other connected services will need an upgrade to IP-based solutions.
As a business owner, it's crucial to understand these changes and explore alternative options to ensure seamless communication for your company's growth.
BTT - ISDN Phaseout from BTT on Vimeo.
Why is ISDN being phased out?
ISDN (Integrated Services Digital Network) has served UK businesses for decades, but it’s now outdated and expensive to maintain.
Modern digital alternatives — particularly VoIP (Voice over Internet Protocol) — offer:
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Faster setup
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Lower running costs
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Greater flexibility
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Advanced features like call analytics and remote access
BT and other network providers are retiring ISDN to focus on future-ready infrastructure powered by fibre broadband and cloud technologies.
What is replacing ISDN?
Businesses have two primary paths forward:
1. SIP Trunking (Session Initiation Protocol)
SIP allows you to retain your existing office phone system while replacing ISDN lines with internet-based voice connections.
Why businesses choose SIP:
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Cost savings vs. traditional line rental and call charges
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Scalable: Add/remove lines easily
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Great for hybrid setups: If you already have on-premise hardware
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Compatible with many existing PBX systems
To find out more about SIP: click here
Take a look at our ISDN to SIP migration checklist: click here
2. Hosted VoIP (Cloud-Based Phone System)
Hosted VoIP is a cloud telephony solution where your phone system is managed remotely by a provider — no on-site PBX required.
Ideal for businesses that:
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Need flexibility for remote/hybrid teams
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Want to eliminate hardware costs
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Prefer managed services with minimal IT overhead
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Want easy access to features like call forwarding, voicemail-to-email, or call reporting
Learn more about VoIP: click here
Read more about your alternative options: click here
Is Your Business Ready to Migrate?
Switching to a digital voice solution doesn’t have to be complex.
Here's what to consider:
- Is your phone system IP-compatible?
- If yes → consider SIP
- If no → Hosted VoIP may be more efficient
- Is your internet connection strong enough?
- If not, consider upgrading to FTTP or a leased line
- How many users/sites need to migrate?
- Migration timelines vary, but many businesses can transition in as little as 3 weeks
Speak to the Experts:
We’ve helped businesses across industries—from healthcare to education—successfully move from ISDN to cloud-based systems.
Our team offers:
- A free audit of your current setup
- Guidance on the best solution for your needs
- Full support through installation, testing, and training
Contact us now: 0330 222 0330 | info@bttcomms.com | Contact us page | fill out the form below
FAQs
Q: Can I keep my current phone numbers?
Yes — number porting is available for both SIP and VoIP setups.
Q: What if there’s a power cut or internet outage?
We recommend backup systems (like mobile failover or UPS) for continuity.
Q: Will the call quality be as good?
Absolutely. With a stable fibre connection, VoIP often exceeds ISDN in quality.
Q: Is this only for large businesses?
Not at all. SMEs benefit just as much — and often save more per user.
Q: Do I need to change anything else?
Some devices (e.g., alarms, card machines) that rely on phone lines may also need upgrading.