For businesses that have not moved off the ISDN, the transition to SIP is a big one. But when managed well, it can be seamless.
For businesses that have not moved off the ISDN, the transition to SIP is a significant one. But when managed well, switching from ISDN to SIP can be seamless—and unlock major benefits for your organization.
With BT's PSTN and ISDN switch-off deadline now set for January 31, 2027, businesses still relying on legacy telephony infrastructure need to act now. The ISDN to SIP migration isn't just about compliance, it's an opportunity to modernize communications, reduce costs, and gain flexibility that legacy systems simply cannot provide.
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Before diving into the migration steps, let's understand why the ISDN to SIP transition matters. ISDN (Integrated Services Digital Network) served businesses well for decades, but it relies on dedicated physical circuits and aging copper infrastructure. SIP trunking, by contrast, delivers voice and data over IP networks, offering far greater scalability and cost efficiency.
"Migrating to SIP Trunking offers numerous benefits, including cost savings, increased flexibility, and enhanced functionality. With careful planning, the right support, and a clear understanding of the technical requirements, businesses of all sizes can successfully navigate these changes."
Understanding the fundamental differences between ISDN and SIP helps clarify why migration makes strategic sense. Here's how the two technologies compare:
| Feature | ISDN | SIP Trunking |
|---|---|---|
| Infrastructure | Physical copper/fiber circuits with dedicated hardware | Virtual connections over IP networks |
| Scalability | Fixed channels (ISDN2: 2-8 channels, ISDN30: up to 30) | Virtually unlimited, add/remove channels instantly |
| Setup time | 4-6 weeks for new installations | 2-5 days typical deployment |
| Cost model | High CAPEX + fixed monthly fees per channel | Low CAPEX + pay-as-you-go usage |
| Geographic flexibility | Numbers tied to physical exchanges | Numbers portable anywhere with internet |
| Disaster recovery | Single point of failure per circuit | Multi-path redundancy and automatic failover |
| Maintenance | Requires specialized technicians | Remote management via portal or API |
| Future-proofing | Being phased out globally | Foundation for UCaaS and AI integration |
As UC Today notes, the option for instantaneously adding additional trunks as required is what makes SIP trunking such a benefit over standard ISDN.
One of the most compelling reasons to switch from ISDN to SIP is cost savings. Here's a breakdown of the financial considerations:
| Cost Category | ISDN | SIP Trunking |
|---|---|---|
| CAPEX | ||
| Hardware (PRI cards, gateways) | $2,000-$10,000+ | $0-$500 (existing IP-PBX) |
| Installation fees | $500-$2,000 per circuit | Minimal or none |
| Session border controller | Not required | $500-$2,000 (optional) |
| OPEX | ||
| Monthly line rental | $300-$800 per PRI | $15-$25 per channel |
| Call charges (domestic) | $0.02-$0.05/min | $0.005-$0.01/min |
| International calls | High per-minute rates | Up to 90% lower |
| Maintenance contracts | $100-$500/month | Often included |
| 3-Year TCO | Higher | 50-70% lower |
According to net2phone's analysis, businesses can save up to 50% on line rental and 25% or more on call expenses when migrating from ISDN to SIP.
Begin your ISDN to SIP migration by documenting your existing setup:
This audit forms the foundation for your migration planning. For multi-site organizations, Telnyx's SIP trunking architecture guide explains how SIP can consolidate connections across locations.
SIP trunking requires adequate bandwidth and network quality. Evaluate:
Consider Quality of Service (QoS) configurations to prioritize voice traffic. If your current broadband is unreliable, explore dedicated internet access or SD-WAN solutions before your ISDN to SIP cutover.
Select a provider that offers:
Telnyx operates as a licensed carrier in 30+ countries with a private global network, providing enterprise-grade SIP trunking that matches or exceeds ISDN reliability. For UCaaS providers and enterprises alike, this carrier-grade infrastructure is essential.
Set up your SIP trunking environment following your provider's documentation:
The Telnyx SIP trunking quickstart guide walks through portal setup step by step. For specific PBX configurations, reference the configuration guides covering Asterisk, 3CX, FreePBX, and dozens of other platforms.
Number porting is often the most sensitive part of the ISDN to SIP transition:
"When migrating your telephony, make sure your SIP trunking provider is also a licensed carrier in your locality—or at the very least, has deep regulatory know-how. This gives you access to local telephony assets and a smooth, transparent number porting process."
— Telnyx Product Team, from the ISDN/PSTN Shutdown Migration Guide
Once testing validates your SIP setup:
With SIP in place, you can now explore advanced capabilities like contact center AI, real-time call analytics, and integration with CRM systems.
A well-planned migration follows predictable phases. Here's a typical timeline:
| Phase | Duration | Key Activities |
|---|---|---|
| Discovery & Planning | 2-4 weeks | Audit infrastructure, assess network, define requirements |
| Provider Selection | 1-2 weeks | Evaluate vendors, negotiate contracts, sign agreements |
| Technical Setup | 1-2 weeks | Configure SIP connections, integrate with PBX, initial testing |
| Number Porting | 2-4 weeks | Submit port requests, coordinate with losing carrier |
| Parallel Operations | 2-4 weeks | Run SIP alongside ISDN, extensive testing, staff training |
| Cutover | 1-2 days | Final switchover, decommission ISDN circuits |
| Optimization | Ongoing | Monitor quality, tune settings, explore advanced features |
Total timeline: 8-16 weeks for most organizations. Larger enterprises with multiple sites or complex requirements may need 6+ months.
Businesses that haven't migrated from ISDN to SIP by the deadline will face service termination. In the UK, BT has confirmed that all ISDN services permanently cease on January 31, 2027. Similar shutdowns are occurring globally, Germany has already completed its transition, and Australia's NBN migration continues.
Gartner's research on unified communications confirms that cloud-delivered UC solutions have become the preferred deployment model for organizations of all sizes. The ISDN to SIP migration is simply the first step toward modern, AI-enabled communications.
Don't wait until the deadline approaches. Start your ISDN to SIP transition today:
Related resources:
Ready to make the switch from ISDN to SIP? Contact our team for personalized migration support.
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