SIP Trunking

6 Steps to Follow when Switching from ISDN to SIP

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.

By Fiona McDonnell

6 Steps to Follow When Switching from ISDN to SIP

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.

Why migrate from ISDN to SIP trunking?

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."

— SIP.US Migration Guide

ISDN vs SIP: Key differences

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.

Cost factors: ISDN to SIP migration economics

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.

6 steps to switch from ISDN to SIP

Step 1: Audit your current ISDN infrastructure

Begin your ISDN to SIP migration by documenting your existing setup:

Count all ISDN2 and ISDN30 circuits across locations
Inventory all phone numbers (DDIs) and their usage patterns
Identify special services (fax, alarms, EPOS terminals)
Document PBX make, model, and SIP compatibility
Review current call volumes and concurrent call requirements

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.

Step 2: Assess your network readiness

SIP trunking requires adequate bandwidth and network quality. Evaluate:

Bandwidth: Each concurrent call needs approximately 100kbps
Latency: Should be under 150ms for quality voice
Jitter: Keep below 30ms for consistent audio
Packet loss: Target less than 1%

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.

Step 3: Choose your SIP trunking provider

Select a provider that offers:

Global coverage: Numbers and termination in your operating regions
Carrier-grade reliability: 99.999% uptime SLAs
Flexible authentication: IP, credentials, or FQDN-based options
Transparent pricing: Pay-as-you-go without hidden fees
Migration support: Porting expertise and technical assistance

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.

Step 4: Configure and test your SIP connection

Set up your SIP trunking environment following your provider's documentation:

Create your SIP connection in the provider portal
Configure authentication (credentials or IP whitelisting)
Set up inbound routing for your phone numbers
Configure outbound calling profiles
Test inbound and outbound calls thoroughly

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.

Step 5: Port your numbers and run parallel operations

Number porting is often the most sensitive part of the ISDN to SIP transition:

Submit port requests with accurate customer service records (CSR)
Allow 2-4 weeks for complex ports involving ISDN numbers
Run SIP and ISDN in parallel during the transition period
Test extensively before cutting over completely
Have rollback procedures ready

"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

Step 6: Complete cutover and optimize

Once testing validates your SIP setup:

Schedule cutover during low-traffic periods
Decommission ISDN circuits only after confirming SIP stability
Monitor call quality metrics in your provider dashboard
Optimize codec settings for your bandwidth profile
Train staff on any new features or procedures

With SIP in place, you can now explore advanced capabilities like contact center AI, real-time call analytics, and integration with CRM systems.

ISDN to SIP migration timeline

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.

What happens after the ISDN switch-off?

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.

Next steps for your ISDN to SIP migration

Don't wait until the deadline approaches. Start your ISDN to SIP transition today:

Sign up for a Telnyx Mission Control account
Explore SIP trunking configuration options in the portal
Test with a few numbers before committing to full migration
Contact sales for enterprise migration support and volume pricing

Related resources:

Ready to make the switch from ISDN to SIP? Contact our team for personalized migration support.

Migrating ISDN to SIP? Join our subreddit for tips.

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