Cutting the Cord: Migrating CPA From Legacy ISDN to Microsoft Teams Voice

The Challenge

CPA is a building regulation and member services organisation based in London, with a small, focused team of 15 people. That size is often an advantage - but when it came to telephony, it made the old setup even harder to justify.

CPA was running a legacy on-premises phone system with ISDN voice services installed in their London office. That worked perfectly well in a world where everyone sat at their desks. It did not work in a world of hybrid working.

• Hybrid working exposed the gaps - Extensions had to be manually forwarded to company mobiles every time someone worked remotely, and calls regularly got lost in the process
• No outbound calling from the corporate number - When staff were out of the office, they couldn’t make outbound calls from CPA’s main number, so personal mobiles were being used and shared with members
• Professional image was slipping - Members were increasingly holding personal mobile numbers for CPA staff, which was not a sustainable way to run member communications
• Separate system, separate bill, separate headaches - The ISDN service was a legacy line item that had no business still existing alongside Microsoft 365

Our Approach

CPA already had Microsoft 365 licensing that included Teams. We simply needed to unlock the part of it that replaced the phone system.

Recommending the Right Fit

Our first step was to recommend Microsoft Teams Voice with Direct Routing as a direct replacement for the on-premises PBX. This gave CPA the features they needed - auto-attendant, ring groups, visual voicemail, out-of-office - without adding a new vendor or a new user experience. Staff were already using Teams every day; calls now just lived in the same app.

Migrating Without Disruption

We planned the migration around CPA’s trading hours and member communications, porting numbers, configuring call routing and training staff on the new experience. Because the end-user app was one they already knew, adoption was effortless - there was no “new phone system” to learn, just a new button in a familiar tool.

Adding the Features That Actually Matter

• Ring groups so member calls reach the right team instantly
• Auto-attendant to give members a professional greeting and clear routing
• Visual voicemail so nothing gets missed
• Out-of-office handling integrated with Teams presence
• Consistent outbound caller ID so every call from every CPA staff member - wherever they are - shows the corporate number

The Results

CPA can now work efficiently and professionally from any location with an internet connection. Members get a single point of contact. Staff transfer calls to colleagues cleanly, offer direct-dials for member conversations, and no longer rely on personal mobile devices for work. The legacy on-premises PBX and ISDN lines are gone, and the telephony cost line came down with them.

On top of all that, CPA now has something they never had before: real reporting. We deliver call and user experience reporting that gives CPA visibility into their phone system as a service - how it’s performing, where the pressure points are, and what’s actually happening on the line.

Looking Forward

We continue to manage CPA’s Teams Voice environment as part of their ongoing Microsoft 365 service, refining call flows as the business evolves and keeping the reporting in front of the people who make decisions about member experience.