In today’s fast-paced business environment, the transition from traditional telephone lines to internet-based solutions is well underway. SIP trunking has emerged as the leading alternative to ISDN, offering flexibility, scalability, and cost-efficiency. With the UK’s planned switch-off of the Public Switched Telephone Network (PSTN) and ISDN services by the end of 2025, businesses must act quickly to modernise their telephony systems. Here’s a breakdown of ISDN vs SIP to help you understand which is the better option for your organisation.

Circuits and Hardware

ISDN relies on physical infrastructure, with BT engineers installing boxes and cables throughout a building. Each ISDN30 circuit provides 30 channels, while ISDN2 circuits come in increments of two channels. For example, an eight-channel ISDN2 setup requires four small boxes. This hardware-heavy approach can be cumbersome and limits flexibility.

SIP trunking, by contrast, operates virtually. It doesn’t require on-site hardware for call termination, instead leveraging your existing internet connection. The number of channels you can support depends on your available bandwidth, making SIP more adaptable and easier to implement in modern office environments.

Scalability

Scaling with ISDN can be time-consuming and costly. Adding channels requires new hardware installations, and ISDN2 upgrades come in fixed increments of two channels. For ISDN30, the pipe allows for 8–30 channels to be unlocked, but anything beyond that requires additional circuits.

SIP, however, offers virtually unlimited scalability, provided your internet connection has sufficient bandwidth. Adding or reducing channels is as simple as adjusting your subscription, making it an ideal solution for businesses that anticipate growth or seasonal fluctuations in demand.

Backup and Redundancy

ISDN offers limited backup options. While you can divert calls to mobile or external numbers during outages, it lacks robust redundancy features. Hardware failures in ISDN systems often require significant time and effort to repair, leaving businesses vulnerable to extended downtime.

SIP trunking provides more robust backup options. With a secondary internet connection, such as a backup Ethernet line, businesses can maintain continuous connectivity. SIP’s flexibility allows calls to be automatically rerouted to other devices or locations in case of a failure, ensuring minimal disruption to operations. Properly configured firewalls and redundant systems further enhance network reliability.

Cost

ISDN involves a monthly charge per channel, along with installation fees and ongoing costs for additional features like Direct Dial-In (DDI) numbers and Caller Line Identification (CLI). Maintenance and hardware upgrades add to the overall expense.

SIP offers a more cost-effective solution. While there may be an initial setup fee, monthly costs are typically lower, with fees based on the number of trunks required. SIP eliminates many hardware-related expenses, making it more affordable in the long run.

Capacity and Planning

ISDN requires careful planning to ensure sufficient capacity, especially for businesses with fluctuating communication needs. Expanding capacity involves lead time for hardware installation and configuration.

With SIP, capacity adjustments are quick and straightforward. Additional channels can be added almost instantly, allowing businesses to scale their telephony system up or down as needed without extensive planning or downtime.

Key Takeaways: ISDN vs SIP in 2025

1. PSTN and ISDN Switch-Off: With ISDN being phased out by the end of 2025, businesses still relying on it must transition to SIP or other modern telephony solutions.

2. Flexibility and Scalability: SIP offers greater adaptability, allowing businesses to easily adjust their communication capacity without hardware upgrades.

3. Cost-Effectiveness: SIP trunking reduces costs by eliminating hardware dependencies and offering more affordable subscription models.

4. Reliability: SIP provides better redundancy and backup options, ensuring uninterrupted connectivity.

   
   

Next Steps

As the deadline for ISDN approaches, businesses must act now to avoid service disruptions. Intouch Communications can help you make the switch to SIP trunking, ensuring a smooth transition and optimised telephony solutions tailored to your needs.

Contact us today via our enquiry form or call us on 01252 241000 to explore how SIP can revolutionise your business communications in 2025 and beyond.