As the UK ditches old-school phone lines, many organisations are ditching ISDN too, switching to things like VoIP, SIP trunking, or hosted phone systems. While making the jump usually means more flexibility and lower long-term costs, businesses usually hit a few hurdles during migration.
This blog post will help you get a grasp of these common ISDN replacement issues, to help you understand your infrastructure and avoid disruption.
1. Internet Connectivity Not Being Strong Enough
ISDN lines operate independently of your internet connection, while modern VoIP systems rely entirely on it. Businesses with low bandwidth strength, unstable connections, or poor network configuration may experience:
- Dropped calls
- Audio latency
- A drop in call quality
It’s imperative to have a dedicated business-grade connection or QoS configuration.
2. Network Infrastructure Not Being VoIP Ready
It’s no surprise that lots of businesses struggle when replacing ISDN. This is because they were never designed to handle voice traffic in real time.
A common trait among offices is that their networks tend to put general data usage first, while VoIP only requires a stable connection, low latency and consistent packet delivery to maintain clear call quality.
Users need to ensure their underlying infrastructure is ready. If not, voice traffic fights for space with things like cloud applications, large downloads or video calls. This can cause VoIP issues such as jitter, latency and dropped calls.
Today’s VoIP environments are built to prioritise performance. This way, businesses can keep communications during busy times.
The key elements of a VoIP infrastructure are:
- Enterprise-grade routing and switching designed to handle voice traffic efficiently
- Quality of Service (QoS) to prioritise voice packets over normal data traffic
- Network segmentation (VLANs) to separate voice and data networks
- Low-latency connectivity to maintain consistent call quality
- A scalable infrastructure that can handle calls as the business grows
When users have the right infrastructure, businesses can move away from ISDN with confidence. This ensures their VoIP system provides high-quality communication across the organisation.
3. Legacy Phone Systems That Cannot Integrate
Many businesses still operate legacy PBX systems that were built specifically for ISDN.
When switching to modern telephony, companies often discover:
- Hardware incompatibility
- Expensive upgrade paths
- Limited SIP support
In many cases, replacing the entire phone system becomes the most practical option.
4. Bandwidth Miscalculations
VoIP calls require predictable bandwidth.
A single HD voice call typically requires around 100 kbps both ways, and businesses with multiple simultaneous calls can quickly exceed available capacity.
Without accurate planning, companies may experience congestion during busy periods.
5. Poor Call Quality During Migration
When a business first switches from ISDN to VoIP, they usually have a couple of problems with call quality at the beginning. Contrary to popular belief, these problems are generally because of the way the network is set up, not because of the VoIP system itself.
Since VoIP is powerless without the internet, the voice data has to move smoothly across the network. Now, if the network is not set up correctly for voice calls, this could seriously affect the quality of the call.
Some common reasons for bad VoIP call quality are:
- Packet loss – This is when parts of the voice data do not reach the person on the end.
- Network jitter – This is when the voice data packets do not arrive at the same time, which makes the sound choppy or distorted.
- Incorrect codec setup – This is when the voice data is not encoded correctly, which affects how clear the sound is and how much bandwidth it uses.
Fortunately, these issues are a quick-solve just by setting up the network correctly, making sure the important traffic gets through first and setting up the codec correctly. Once everything is set up right, VoIP systems can give call quality that’s as good as or even better than traditional ISDN connections. VoIP systems are really good at making sure calls sound clear and strong. They can do this because they are set up to work well with the internet. VoIP systems and the internet work together to make sure calls are of good quality.
6. Security Concerns with VoIP
Internet-based phone systems are different from ISDN. They work over IP networks. This means they can be at risk if they are not set up correctly.
Potential threats include:
- SIP registration attacks, where people who are not supposed to use the phone system try to get in
- Toll fraud, where people who have gotten into the system make expensive international calls
- DDoS disruptions, which can cause the system to be too busy and affect how well calls work
To make these risks smaller, businesses should put good security measures in place when they stop using ISDN and start using VoIP. This usually means having firewalls that are set up well, session border controllers, rules for who can use the system and people watching the network.
If you have the security, VoIP systems can work safely and give you the flexibility and ability to grow that you need for your business communications.
7. Staff Training and Adoption
Moving from traditional desk phones to cloud systems introduces new features such as:
- Softphones
- Mobile apps
- CRM integrations
- Video conferencing
Without training, staff may struggle to adapt to new workflows.
8. Porting Existing Phone Numbers
Porting your numbers is one part of moving away from ISDN that you really don’t want to mess up. One small slip and you could be looking at temporary service disruption, lost inbound calls, or customers being unable to reach key departments.
Common issues include:
- Incorrect account information was submitted to the losing carrier
- Multiple telecom providers are involved in the number range
- Long lead times when porting large blocks of numbers
- Rejected port requests due to mismatched billing details
If these problems are not identified early, the result can be missed calls, operational disruption, and delays to the planned ISDN switch-off migration.
At Fuse 2, we use a combination of automated validation and AI-assisted checks to reduce these risks before a port request is submitted. It’s a proactive way to dodge the common errors that trip most people up, drastically improving the chances of a successful, headache-free transfer.
9. Business Continuity Planning
ISDN systems were often tied to physical lines that remained operational during internet outages.
VoIP systems require alternative resilience strategies such as:
- Automatic call routing
- Mobile failover
- Cloud redundancy
This ensures businesses remain reachable even during connectivity issues.
10. Choosing the Right ISDN Replacement Solution
Lots of companies have a time figuring out which technology to use when they stop using ISDN. Modern business phone systems are different from the phone networks. They have different ways of doing things, and each one is good for different kinds of setups and ways of working.
To pick the one you need to think about the phone system you already have, how good your internet is, how big your company is and how your team talks to each other every day. If you know what the common options are, then you can make a better choice when you decide to stop using ISDN.
SIP Trunking for Existing PBX Systems
For businesses that already have a PBX system at their office, using SIP trunking is usually the way to replace their old ISDN system.
SIP trunks get rid of the physical ISDN lines. Replace them with connections that go through the internet.
This means businesses can keep using their phone system and still get the good things that come with new technology.
The good things about SIP trunks are:
- Lower costs for phone calls compared to what they pay for ISDN
- Businesses can get phone lines without having to install new lines
- SIP trunks work with a lot of the old PBX systems that are already out there
- It is faster to set up phone lines
This is a good choice for companies that just bought new PBX equipment and want to make it last longer, without having to switch everything over to the cloud.
Hosted VoIP Phone Systems
A hosted VoIP system gets rid of the need for on-site PBX hardware
The phone system is now hosted in the cloud. You can access it through desk phones, laptops or mobile devices. This helps businesses manage their communications through an online platform.
Hosted VoIP systems are getting more popular. They offer benefits, including:
- Greater flexibility for hybrid working with hosted VoIP systems
- Reduced hardware maintenance with hosted VoIP
- Easy system management through web portals for hosted VoIP
- Built-in features like call recording and analytics in hosted VoIP systems
For modern organisations, hosted VoIP provides a simpler and more scalable long-term alternative to traditional PBX systems.
Organisations use hosted VoIP systems to improve their communication.
Some businesses adopt communications (UC) platforms.
These systems combine communication tools into one platform, including:
- Voice calls
- Video conferencing
- Instant messaging
- File sharing
- CRM integrations
Unified communications solutions aim to improve collaboration across teams. They reduce the need for separate tools.
Organisations with distributed teams or high collaboration requirements use UC platforms.
These platforms streamline external communication for organisations.
Organisations use UC platforms to improve team collaboration with communications.
Unified communications solutions help organisations work better together.
How Fuse 2 Can Help
The move away from ISDN is a big deal for business telecommunications. It has been a long time since things have changed this much. The change to VoIP or SIP trunking can be tricky from a point of view. It is also a chance to update the way your organisation talks to each other.
At Fuse 2, we help businesses deal with the end of ISDN. We do this by creating a plan to move to the new system. We look at the internet and the equipment you already have. Then we set up SIP trunks and VoIP systems. Our team makes sure that everything goes smoothly and that your business does not get disrupted.
The new cloud telephony systems are a lot better than a replacement for ISDN. Businesses can now. Change more easily. People can work from home or other places. It is easy to manage calls. The costs of running the system are also a lot lower. This is what businesses get from the cloud telephony systems. The ISDN switch-off is a change but it is also a good opportunity for businesses to improve the way they communicate with the help of ISDN switch-off.
