Migrating from Legacy PBX to 3CX: A Complete Transition Planning and Execution Checklist

For IT directors and business owners in the UK, the decision to migrate from a legacy PBX (like an ageing Mitel, Avaya, or Siemens/Unify system) to a modern IP-based solution like 3CX is a significant strategic move. The motivations are clear: reduced costs, remote work enablement, and powerful features like CRM integration. But the biggest driver of all is the impending 2027 PSTN and ISDN switch-off, which makes this migration a matter of when, not if.

The core challenge is simple: how do you completely replace your company's "dial tone" without causing a catastrophic, business-wide outage? As our The Ultimate 3CX Guide for Business: Setup, Pricing, Features, and Best Practices explains, this is a critical undertaking. This guide provides the complete, exhaustive checklist for planning and executing that migration with a focus on the UK market, covering the porting process, minimising downtime, and crucial post-migration checks.

Pre-Migration Checklist: Licensing, Network Readiness, and User Data Audit

Success in a PBX migration is 90% planning. The "cutover" day should be the fast, boring conclusion to a long, meticulous preparation phase.

1. 3CX Licensing and Deployment

Determine Simultaneous Calls (SC): This is your most critical licensing metric. It's not your user count. A common rule of thumb is 1 SC for every 3-4 users. A 100-user company will likely need a 24 or 32 SC licence.

Select Your Host: Where will 3CX live?

• On-Premise: On a new server or VM (Hyper-V/VMware) in your existing server room.
• Cloud-Hosted: In your own private cloud instance (AWS, Azure) or using 3CX's hosted solution.

2. Network Readiness Assessment

Run the 3CX Firewall Checker: Your 3CX server (on-prem or cloud) must have all its ports correctly configured. Run the Firewall Checker in the admin console and ensure it passes with all green checkmarks. This is non-negotiable.

Assess Bandwidth: A modern G.711 (high-quality) voice call uses about 80-100 kbps per call.

This is typically not an issue for data, but you must ensure your firewall's Quality of Service (QoS) rules prioritise this (RTP) traffic to prevent jitter and choppy audio.

Check Local Network: Your local switches must have PoE (Power over Ethernet) to power the new IP phones. If not, you must order PoE injectors or new PoE-capable switches.

3. Hardware and Endpoint Audit

• Order New Phones: Your old digital Mitel/Avaya/Siemens phones are not compatible with 3CX. You will be buying new SIP-based IP phones (e.g., Yealink, Fanvil, Poly).
• Order SBCs: For any remote offices, you must order hardware for the 3CX Session Border Controller (SBC). The most common choice is a Raspberry Pi 4 for each remote site.
• Headsets: Audit all users. Who needs a headset? Are they USB (for softphones) or QD (for desk phones)?

4. User and Call Flow Data Audit

This is the most time-consuming but crucial part of the planning. You must completely document your current system to replicate its functionality in 3CX.

Call Flow "Discovery": How do calls actually get into your company?

• Main Number: What happens when someone calls the main number? Does it hit an IVR/Auto-Attendant? ("Press 1 for Sales..."). Document every option and its destination.
• DIDs: List every phone number your company owns. Where does it route? To a user? To a ring group?
• Ring Groups: Document all hunt groups. Who is in them? What is the ring strategy?
• Voicemail: Do you have a "general" voicemail box?
• Business Hours: What are your open/closed hours? What happens to calls "after hours"? What is your holiday schedule?

Detailed Process for Porting Phone Numbers and Configuring SIP Trunks

This is the scariest part of the migration for most businesses. Porting is the "legal" process of moving your phone numbers from your old carrier (e.g., BT, TalkTalk) to your new SIP trunk provider.

Step-by-Step Porting and SIP Trunk Process:

1. Choose a 3CX-Supported UK SIP Trunk Provider

You must sign up with a new SIP trunking provider. This is the company that will deliver your calls over the internet to your 3CX PBX. Your old ISDN lines will be made redundant.

UK Examples: Gamma and other popular, 3CX-compatible UK providers.

2. Submit the Porting Request (LNP)

You will submit a Letter of Number Porting (LNP) or "Port Order" to your new SIP provider. Learn more about how phone number porting works.

Required Info: You must provide them with:

• A list of all numbers to be ported.
• A recent (within 30 days) copy of the bill from your old provider.
• The service address and account/PIN numbers from the old bill.

3. The Waiting Game and FOC Date

Your new provider submits the port request to your old provider (e.g., BT Openreach). This takes time (anywhere from 1 to 4 weeks).

Eventually, the providers will agree on a FOC (Firm Order Commitment) Date. This is the exact date and time (usually a specific 4-hour window) when the port will be executed.

4. Configure Your New SIP Trunks in 3CX

While you wait for the FOC date, your new SIP provider will give you SIP trunk credentials (e.g., server address, auth ID, password).

Action:

1. Go to your new 3CX server > SIP Trunks > Add SIP Trunk.
2. Select your provider's template (e.g., "Gamma").
3. Enter your authentication details.
4. The trunk will register (show green).

Best Practices for Minimising Downtime During the Switchover

Downtime is not inevitable. With the "parallel build" strategy, you can achieve a "flash cut" migration with less than 5 minutes of total downtime.

The Parallel Build Strategy

You have your old Mitel system, and it's 100% functional. You also have your new 3CX server, 100% functional, using its temporary numbers. You will use the 2-4 weeks of port-waiting time to build a perfect clone of your old system inside 3CX.

The "Flash Cut" - The Day of the FOC

Your FOC date is today at 2:00 PM. Here is the plan:

Post-Migration Audit and User Training Considerations

The cutover is done, but the project is not over.

1. Immediate Post-Migration Audit

For the next 24-48 hours, you are in "hyper-care" mode.

Test Every DID: Get your original list of all ported DIDs. Call every single one from an external phone (e.g., your mobile). Does it route to the correct user/IVR/queue? This is tedious but you will find a typo or a missed rule.

Test All Outbound Rules: Have users from different departments (Sales, Support) make outbound calls to test.

Test Emergency Services (CRITICAL):

• The emergency numbers in the UK are 999 and 112.
• DO NOT dial 999 as a test without authorisation.
• The Correct Procedure: To arrange a test, you must email 999testcalls@bt.com. They will provide you with a test window and a script to follow. This is the only 100% legal way to test a 999 call.
• Non-Emergency Test: You can (and should) test the 101 (police non-emergency) number to verify 3-digit dialling and routing.
• E999 Address: Verify with your SIP provider that your E999 (Enhanced 999) address is correctly registered for your main number, so emergency services see the correct location.

2. User Training (This should not wait until post-migration)

Your users have been using the same Mitel phone for 15 years. They do not know how to use a modern VoIP phone, and they especially don't know the power of the 3CX Web Client.

Focus on the Clients: Do not waste time teaching them the buttons on the desk phone. Spend 90% of the training on the 3CX Web Client and Mobile App.

Conclusion: A Successful Migration is a Planned Migration

Migrating from a legacy PBX to 3CX is a transformative project that will modernise your business communications, especially with the 2027 ISDN switch-off looming. The risk of failure is real, but it is 100% manageable. By meticulously auditing your current environment, building your new 3CX system in parallel, and managing the porting process with precision, you can execute a "flash cut" migration with near-zero downtime. The final, critical ingredient is user training—a well-deployed system is only successful if your team knows how to use it.