How to retire the legacy PBX without losing local calling, analogue devices, paging, door entry or the infrastructure your sites still depend on
For many organisations, moving users to Microsoft Teams Phone is the easy part. The real complexity often sits behind the scenes. Analogue phones. But also lift and emergency devices, fax machines, door entry systems, paging amplifiers, factory phones, reception devices, existing two-wire cabling, krone frames, and local PSTN circuits. And, perhaps most importantly, the requirement for people at critical locations to continue making and receiving calls if connectivity to Microsoft 365 becomes unavailable.
These are often the reasons an organisation ends up maintaining an expensive legacy PBX long after the majority of its users have migrated to Microsoft Teams.
The PBX may have hundreds of unused extensions, consume rack space, require specialist support and represent another platform that needs patching, maintaining and renewing. Yet it remains because five analogue devices still need dial tone, a warehouse needs paging or a site cannot accept losing local telephony during a WAN outage.
It does not have to be this way.
With the right combination of Microsoft Teams Phone, Survivable Branch Appliance technology, Ribbon Session Border Controllers, analogue gateways, Algo endpoints and local engineering, organisations can build a Microsoft-first voice architecture without ignoring the physical and operational realities of their sites.
This is exactly where Aura’s Integrate to Migrate approach comes in.
Real enterprise environments rarely look that simple.
A site survey may uncover:
Historically, the easiest response was often: leave the PBX there.
That solves the immediate problem, but it leaves the customer operating two voice environments indefinitely. Licensing, hardware maintenance, power, rack space, support contracts and specialist knowledge remain. More importantly, the organisation never quite completes its migration.
A better approach is to understand why the PBX still exists and replace those individual functions with technology designed for a modern Teams environment.
One of the most important tools for organisations with critical sites is the Microsoft Teams Survivable Branch Appliance, or SBA.
Microsoft developed SBA functionality for Direct Routing environments where a site may temporarily lose connectivity to Microsoft 365 while the local network and connection to the site’s Session Border Controller remain available.
When properly designed and configured, Teams clients at the affected branch can automatically enter appliance mode and continue to make and receive PSTN calls through the local SBC. When connectivity to Microsoft is restored, users automatically return to normal Teams operation.
That changes the conversation around cloud voice.
Previously, organisations could look at an on-premises PBX and a cloud calling platform and ask:
“What happens to the site if the cloud disappears?”
An SBA allows us to design a much more resilient answer.
Microsoft currently supports a range of calling functions in SBA mode, including:
The objective is not to reproduce every Microsoft Teams cloud service locally. It is to preserve the critical voice functionality required to keep a location operational while normal cloud connectivity is restored.
That distinction is important.
Survivability should be designed around the business functions that genuinely need to survive, rather than attempting to recreate the entire Microsoft cloud inside every branch.
Aura works closely with Ribbon to provide the local edge infrastructure required for these more complex environments.
Ribbon’s Microsoft-certified SBC portfolio supports Microsoft Teams Direct Routing and Microsoft SBA, allowing the SBC and survivability architecture to be positioned locally where required. Ribbon supports SBA capabilities across its SBC Edge portfolio, including hardware and virtual deployment options.
But survivability is only part of the story.
Ribbon’s hardware-based edge solutions can also provide traditional interfaces such as FXS, FXO, BRI and T1/E1, allowing modern SIP and Teams environments to interoperate with devices and services that were never designed for Microsoft Teams.
This means the edge can become the bridge between:
Microsoft Teams Phone > SIP > Ribbon > existing analogue or traditional telephony infrastructure
Instead of maintaining an entire PBX to support a handful of devices, we can provide the specific interfaces that those devices actually require.
Not necessarily. This is one of the biggest misconceptions we see during Teams migrations.
An analogue telephone, fax machine, door entry unit or other device does not necessarily care whether the organisation’s core communications platform is Microsoft Teams, Avaya or a traditional PBX.
It needs an appropriate analogue interface and dial tone.
With an FXS interface or suitable analogue gateway, we can convert between SIP and the analogue connection required by the endpoint.
That means an organisation could have:
Microsoft Teams Phone > Ribbon SBC / analogue gateway > existing two-wire cabling > analogue device
This is where telecommunications experience becomes just as important as Microsoft expertise.
Many enterprise buildings still contain extensive traditional voice cabling.
You may have 1308 cable leaving the communications room, Krone frames distributing extensions between floors, intermediate distribution points and two-wire connections that eventually appear at a wall socket hundreds of metres away.
Moving to Microsoft Teams does not automatically mean all of this cabling has to be removed.
Where the cabling is serviceable and appropriate for the application, Aura can assess whether it can be reused.
Our engineers can:
This is an important difference between delivering a Teams licence and delivering a complete communications migration.
Sometimes the most important part of a Microsoft Teams project involves a screwdriver, cable tester and Krone tool.
Paging is another common reason organisations retain legacy telephony.
Manufacturing plants, warehouses, hospitals, schools, transport environments and large offices may have significant investment in overhead speakers and amplifiers.
Replacing the entire system can be unnecessary.
Using technology from Algo, existing analogue paging infrastructure can be integrated into Microsoft Teams using IP paging adapters. An Algo device can provide the bridge between Teams and an existing paging amplifier, allowing the organisation to retain speakers and other infrastructure while modernising the communications platform behind it.
For new installations, organisations can move further towards IP using Algo speakers, horns, visual alerting and other IP endpoints.
Algo’s supported endpoints can integrate with Microsoft Teams through the Teams SIP Gateway, providing native options for paging, alerting and hands-free visitor communications.
Again, the objective is choice.
Keep what makes sense. Replace what needs replacing. Modernise without creating unnecessary disruption.
The same principle applies to door entry. A legacy PBX may currently provide the extension behind a reception intercom or door phone. Removing the PBX does not mean losing that functionality.
Algo provides IP doorphones, intercoms and door controllers that can integrate into a modern SIP and Microsoft Teams environment, enabling calls from an entrance or intercom point to be presented to the appropriate Teams user or workflow.
The result is a more joined-up communications architecture covering not only knowledge workers at laptops, but also the physical environment around them.
A complete solution might therefore look something like this:
↓ Microsoft Teams Phone
↓ Aura Connect / Direct Routing
↓ Ribbon Session Border Controller + Microsoft SBA at critical locations
↓ Local PSTN or SIP connectivity for survivability where required
↓ FXS / analogue gateways for legacy endpoints
↓ Existing Krone, 1308 and two-wire infrastructure where suitable
↓ Fax, lift phones, emergency phones, analogue handsets and other devices
Alongside this:
Microsoft Teams > Teams SIP Gateway > Algo > paging, alerting, intercom and door entry
The exact architecture changes by location. A headquarters may require a Ribbon SBC, SBA, significant analogue density and resilient local connectivity. A warehouse might require a small analogue gateway and Algo paging integration. A small branch may need nothing more than cloud Teams Phone.
That is why we believe enterprise Teams migrations should be designed site by site, but managed as one global programme.
Architecture is only half the challenge, somebody still has to get into the communications room.
For multinational organisations, that can quickly become a major problem. A Teams migration covering 50, 100 or 500 locations may require people locally to survey equipment, trace cables, install gateways, rack equipment, patch circuits and support cutovers.
Coordinating a different local telecoms company in every country brings the complexity straight back.
Aura Global provides local engineering, deployment and support capabilities across 173 countries, giving organisations one partner through which to coordinate both the cloud transformation and the physical work required at each location.
We can combine central programme management and architecture with local activities such as:
That matters because communications transformation rarely stops at the Ethernet port.
At Aura, our strategy is Microsoft-first. But Microsoft-first does not mean Microsoft-only.
Most enterprise customers have years or decades of communications infrastructure already embedded within their organisation.
→ Our job is not to force everything into the cloud regardless of the consequences.
Our Integrate to Migrate approach is built around connecting Microsoft Teams with the systems, devices and infrastructure that businesses still depend on, allowing customers to modernise at the pace that makes sense for them.
Integrate today. Migrate when you need to.
In many environments, yes. If the only reasons for retaining a legacy PBX are:
then it is worth challenging whether an entire PBX is still the right way to provide those services.
Together, that provides an architecture designed around 100% of the communications requirement, not simply the easiest 90% to migrate to Teams.
Analogue does not always mean simple. Lift phones, fire systems, alarms, emergency help points and other regulated devices may have specific requirements relating to power, signalling, line monitoring, emergency location or local regulations.
They should therefore be assessed individually rather than automatically connected to an analogue adapter.
Microsoft also documents specific limitations that apply when Teams is operating in SBA mode, including considerations around emergency location information and emergency call routing policies.
A proper migration should identify these services during discovery and determine the right technology, connectivity, power and resilience strategy for each one.
That is another reason the discovery phase is so important.
Moving to Microsoft Teams Phone should not mean solving the easy parts and leaving the difficult parts running on an ageing PBX for another decade. The goal should be bigger.
Understand every device. Every circuit. Every critical workflow. Every survivability requirement. Every paging zone. Every door phone. Every cable that still has a reason to exist.
Then design the right destination architecture around them.
At Aura, we bring together Microsoft Teams Phone, Aura Connect, Ribbon, Algo, legacy voice expertise and local engineering across 173 countries to help organisations modernise the complete communications environment.
One partner. One architecture. One accountable outcome.
Integrate to Migrate means moving forward without leaving the difficult parts behind.
Talk to Aura about an Integrate to Migrate assessment covering Teams Phone, survivability, analogue devices, paging, door entry and local site infrastructure.