Session border controllers explained for Microsoft Teams deployments

For years, cloud voice has been framed as the end of telephony complexity. Move calling to the cloud, retire legacy PBXs, and let software handle the rest. For small deployments, that story mostly holds. At enterprise scale, it breaks down fast and session border controllers (SBCs) become crucial.

Voice may now run in the cloud, but it still operates under the same constraints it always has. It is still real time, still exposed to networks outside your control, and still bound by regulatory, security, and reliability constraints that collaboration platforms were never designed to manage.

That is why SBCs never went away.

As enterprises migrate voice to Microsoft Teams and other collaboration platforms, SBCs have become more important, not less. They sit quietly at the network edge, enforcing policy, securing sessions, and making incompatible systems behave predictably.

This article explains what a session border controller is, how it works, and why it remains critical for Microsoft Teams and global voice deployments.

What is a session border controller

A session border controller, or SBC, is a network element that controls how real-time voice sessions are established, secured, and terminated as they cross between networks. Simply put, a SBC is the control layer that makes enterprise VoIP work reliably at scale.

In practice, it sits at the boundary between two voice domains. Most often, that boundary is between:

• An enterprise voice environment and a carrier
• A cloud collaboration platform and the PSTN

The “border” is where problems surface. VoIP traffic relies on SIP signaling and real-time media streams. When those cross administrative, security, or geographic boundaries, things break. IP addresses change. Firewalls interfere. Standards are interpreted differently by vendors and carriers.

The SBC exists to absorb that friction.

Within enterprise voice architecture, a session border controller:

• Shields internal systems from external exposure
• Normalizes SIP signaling between platforms
• Controls media paths and call routing
• Enforces security and policy

Physically, an SBC is not a single thing. In some environments, it is a dedicated hardware appliance racked in a data center, sitting alongside firewalls and routers. In others, it runs as a virtual machine or cloud-native service, invisible to users but performing the same role. The form factor changes. The function does not.

How a session border controller works in VoIP networks

A session border controller operates on two parallel planes: signaling and media.

On the signaling side, the SBC manages SIP traffic. It terminates sessions, validates messages, and rewrites signaling so incompatible systems can understand each other. This matters because SIP standards are implemented differently across platforms. Without mediation, calls fail for reasons that are difficult to trace.

On the media side, the session border controller manages the real-time audio streams that carry voice. It anchors those streams so calls can reliably pass through firewalls and network address translation systems. When endpoints do not support the same audio formats, it can translate between them. It also monitors packet loss, jitter, and latency to protect call quality while calls are in progress.

Policy ties everything together. The SBC determines:

• Which carrier a call uses
• How failover behaves when routes degrade
• Which regions handle specific call types

The result is interoperability. Cloud platforms, legacy PBXs, and multiple carriers behave as one voice environment. Without an SBC, enterprises are left trusting that standards behave consistently. They rarely do.

Why SBCs are important in VoIP networks

VoIP exposes enterprises to risks that traditional data networks do not handle well.

• Security is the most obvious. SIP traffic is routinely scanned, spoofed, and exploited. An SBC terminates external sessions before they reach internal systems, blocking malicious traffic at the edge.
• Call quality is the second. Voice is unforgiving. Delays and packet loss are immediately audible. SBCs understand real-time media in ways general firewalls do not.
• Reliability follows. Enterprises expect dial tone to work even when parts of the network fail.

SBCs support:

• Geographic redundancy
• Multi-carrier failover
• Automated rerouting

Then there is scale. As organizations expand globally, voice environments become harder to manage. SBCs provide a consistent control layer as users, locations, and carriers multiply.

This is why SBCs are important in VoIP networks. They are not legacy infrastructure. They are what turns internet-based calling into something enterprises can rely on.

SBC voice security and compliance considerations

Voice security is about more than availability. It is about protecting conversations, preventing fraud, and meeting regulatory obligations.

An SBC plays a central role in voice security by controlling both signaling and media. It enforces encryption for calls in transit, preventing interception and downgrade attacks. It also detects abnormal call patterns and blocks unauthorized destinations.

From a compliance perspective, SBCs support:

• Call recording and lawful intercept routing
• Regional breakout for data residency
• Detailed logging for audit readiness

Just as important is visibility. SBCs generate call detail records, quality metrics, and security logs. When something goes wrong, they provide the evidence trail.

Without an SBC, enterprises struggle to prove that their voice traffic is secure, compliant, and under control.

Session border controllers in Microsoft Teams environments

Microsoft Teams simplifies collaboration, but telephony still needs careful design.

That is why Microsoft mandates the use of certified session border controllers for Direct Routing. In a Teams environment, the SBC sits between Microsoft’s cloud and external telephony networks. It terminates carrier SIP trunks, normalizes signaling, and controls how media enters and exits the Teams platform.

This separation is intentional. Teams focuses on user experience and collaboration. The SBC handles telephony control.

With an SBC in place, enterprises gain capabilities that Teams does not provide natively, including:

• Custom call routing logic based on geography, cost, or time of day
• Multi-carrier architectures for resilience and redundancy
• Country-specific PSTN connectivity to meet regulatory requirements

These capabilities become increasingly important as deployments scale. Native Teams Calling options are designed for simplicity, not for complex global voice environments where routing decisions matter.

Legacy integration is another major driver. Many organizations still rely on on-premises PBXs, analog devices such as elevators and fax machines, or established contact center platforms. The SBC allows these systems to coexist with Teams, enabling gradual migration instead of disruptive cutovers. Calls can flow between old and new environments without users needing to think about the underlying complexity.

For global deployments, this role is essential. Microsoft does not provide PSTN coverage in every country, and even where it does, local regulations often require calls to be routed through in-country carriers. The SBC bridges those gaps, connecting Teams to the right carrier in each region while maintaining a consistent dialing experience for users.

An SBC Teams deployment is not about adding technology for its own sake. It is about retaining control over routing, resilience, and compliance as Teams becomes the primary voice platform.

Types of session border controllers

Session border controllers are deployed in three common models. The underlying function is the same in each case. What changes is where the SBC runs and who is responsible for operating it.

• On-premises SBCs are typically deployed as physical or virtual appliances inside an organization’s own data centers. They offer the highest level of control over security policies, routing logic, and network integration. This model is common in regulated industries or environments with strict data sovereignty requirements. The trade-off is operational overhead. Capacity planning, redundancy, upgrades, and ongoing maintenance all sit with the enterprise.
• Cloud-based SBCs run in public or private cloud environments. Functionally, they behave much like on-premises systems, but without the need for local hardware. This model suits organizations that are moving away from data centers while still wanting direct control over routing and policy. It reduces infrastructure management, but the enterprise still owns the design and day-to-day operation.
• Managed SBC services remove the platform from the enterprise’s responsibility altogether. A provider deploys, certifies, monitors, and maintains the SBC on the organization’s behalf. This approach is increasingly common for global Microsoft Teams deployments, where consistency across regions matters more than owning the infrastructure. Enterprises retain architectural control, but not the operational burden.

The difference between these models is not capability. All can support enterprise voice at scale. The real distinction is ownership. Each model solves the same technical problems. They simply differ in who is accountable for keeping the platform running.

Pure IP supports all three models, offering on-premises, cloud-based, and fully managed session border controller options to match different regulatory, operational, and global deployment requirements >>

Enterprise scenarios where an SBC is essential

Some voice environments simply cannot function without a session border controller. The complexity is not theoretical. It shows up as failed calls, inconsistent routing, and gaps in compliance.

Global, multi-country deployments are the most common example. Each country introduces:

• Different carriers and interconnect requirements
• Local numbering plans and dialing rules
• Regulatory constraints around routing and data residency

An SBC provides a central control point that allows calls to be routed locally where required, while still presenting a consistent dialing experience to users.

Hybrid environments are another. Many enterprises are not starting from a clean slate. They are running Microsoft Teams alongside:

• Legacy on-premises PBXs
• Analog devices such as elevators and alarms
• Established contact center platforms

An SBC enables coexistence, allowing traffic to flow between platforms while migrations happen gradually instead of all at once.

Resilience-led architectures also depend on SBCs. Enterprises with low tolerance for outages use them to support:

• Multi-carrier voice designs
• Geographic redundancy
• Automatic rerouting during carrier or network failures

The SBC is where routing decisions are made when call quality degrades or a carrier goes offline.

Compliance-driven industries add another layer. Healthcare, financial services, and public sector organizations require:

• Controlled call routing by region
• Integration with recording and monitoring systems
• Auditable logs and call detail records

In these scenarios, the SBC is not optional. It is foundational.

Common misconceptions about session border controllers

Several misconceptions about session border controllers continue to surface, particularly as enterprises accelerate cloud voice adoption. Most of them come from treating voice as just another software workload.

1. SBCs are only for legacy voice

This is the most common mistake. In reality, cloud-first Microsoft Teams deployments often depend on SBCs more than traditional PBX environments. As soon as Teams needs to connect to external carriers, support multiple regions, or integrate with existing systems, an SBC becomes the control point that holds the architecture together.

2. Cloud voice removes the need for SBCs

Cloud platforms simplify endpoints and user experience. They do not eliminate the complexity of telephony. Routing, security, compliance, and carrier interconnection still exist. The SBC does not disappear. It simply moves to a different place in the architecture.

3. All SBCs are the same

They are not. Differences in certification, scalability, security capabilities, and operational models have real consequences at enterprise scale. An SBC certified for Microsoft Teams behaves very differently from a generic SIP gateway, and managed services operate differently from self-hosted platforms.

These misconceptions tend to surface only after voice breaks at scale, when outages, routing failures, or compliance gaps force a redesign that could have been avoided earlier.

The session border controller remains core enterprise voice infrastructure. Not because vendors insist on it, but because global VoIP still demands control, security, and interoperability.

For Microsoft Teams deployments, SBCs bridge collaboration software and real-world telephony. They secure calls, normalize complexity, and enable routing flexibility cloud platforms alone do not provide.

Voice may now be software-driven, but its requirements have not changed. Security. Reliability. Compliance. Scale.

SBCs quietly make all of that possible. And for IT leaders building global Teams environments, they remain one of the most consequential architectural decisions they will make.

If you are planning or running a Microsoft Teams voice deployment, SBC decisions matter. Pure IP helps enterprises choose, deploy, and operate the right session border controller model for their environment, whether on-premises, cloud-based, or fully managed.

Get in touch to discuss how we can help your SBC architecture support scale, security, and global coverage.