Cloud SBC explained: Microsoft Teams direct routing

If you’ve ever wondered how seamless communication is maintained in Microsoft Teams— especially when dealing with external calls and complex network environments—we have the answer: session border controllers (SBCs).

SBCs are the unsung heroes of ensuring secure, reliable connectivity for your Teams experience. In this blog post, we'll explain what SBCs are and how they integrate with Microsoft Teams to provide a smooth communication landscape. Whether you're a tech enthusiast, an IT professional, or simply curious about the backbone of your Teams calls, this post will equip you with a comprehensive understanding of SBCs and how you can deploy them in your Teams environment.

Most Teams users may never make an external call

Microsoft Teams has become the default collaboration hub for hundreds of millions of workers worldwide. But there's a significant gap between how many people use Teams and how many can actually dial outside of it — and that gap has real consequences for how organizations should architect their communications.

Teams' scale is hard to overstate: the platform crossed 320 million monthly active users as of Microsoft's FY24 Q1 earnings. Yet according to UC Today, only 26 million of those users are PSTN-enabled — roughly 8% of the total user base. These are the users who can make and receive calls to standard phone numbers through Teams Phone.

That 8% figure is growing — PSTN-enabled Teams users have increased 30% over the past 20 months, according to Cavell Group research cited by UC Today in December 2025. But the gap between collaboration-only adoption and full telephony integration points to something most IT teams already know: for a majority of organizations, Teams still coexists with a separate phone system rather than replacing it entirely.

For those making the move to full PSTN integration through Teams, infrastructure reliability becomes the central question. Outages happen across every provider, and a single disruption can cut off external communications at scale. The difference between a seamless failover and a prolonged blackout often comes down to how the underlying architecture was built.

The architecture decision at the heart of reliable Teams calling is the session border controller. Understanding what an SBC does — and how it's deployed — is where most of the complexity lives.

What is a session border controller?

A session border controller (SBC) is a network device or software application used in Voice over Internet Protocol (VoIP) and Unified Communications (UC) networks to manage and control the flow of communication sessions, such as voice calls, video calls, and other real-time media sessions. SBCs play a crucial role in ensuring the security, interoperability, and quality of these sessions.

You can think of an SBC as a guardian angel for your VoIP communications, managing and safeguarding SIP (Session Initiation Protocol) connections. More than just a call connector, it's about ensuring those calls are of exceptional quality, highly secure, and fully compliant with regulations.

Why you should add an SBC to your Microsoft Teams setup

SBCs are a requirement when connecting to Teams. However, the specific reasons for choosing to do so depend somewhat on your use case and the technology you plan on using. Here are some common examples:

Telephony integration and PSTN connectivity

SBCs enable integration between Microsoft Teams and traditional telephony systems, such as the Public Switched Telephone Network (PSTN). This integration allows users to make and receive phone calls from within the Teams interface, effectively replacing or complementing traditional phone systems.

SBCs are also often used to connect Microsoft Teams to the PSTN, allowing users to make external calls to regular phone numbers. This connection is essential for businesses that need to maintain external communication capabilities.

SIP trunking

Organizations using Session Initiation Protocol (SIP) trunks for voice communication can employ SBCs to connect these SIP trunks to Microsoft Teams, facilitating voice communication over the SIP protocol.

Security

SBCs enhance the security of Microsoft Teams communication, serving as a security barrier and protecting against unauthorized access, eavesdropping, and various cyber threats. SBCs can also enforce security policies, encrypt communication, and provide robust access control.

Compliance

Some industries have specific regulatory compliance requirements related to communication and data retention. SBCs can assist in meeting these compliance requirements by providing features like call recording, call monitoring, and call logging.

While there are many reasons you should add an SBC to Teams, your decision doesn’t stop there. You’ll also have to figure out which approach to SBCs will work best for your organization.

Cloud SBC adoption by the numbers

The enterprise communications market is shifting rapidly to the cloud, and session border controllers are at the center of that transition. These figures illustrate the scale of the opportunity.

320 million monthly active Microsoft Teams users as of 2024 (Microsoft), with the majority requiring PSTN connectivity via direct routing to handle external calls.

$2.6 billion — projected cloud SBC market size by 2027, growing at a 15.7% CAGR as enterprises replace on-premises hardware with carrier-grade cloud infrastructure (Grand View Research, 2023).

40% average cost reduction organizations achieve when switching from Microsoft Calling Plans to direct routing via a cloud SBC, by leveraging competitive carrier rates (industry analysis, 2024).

David Casem, CTO @ Telnyx

That architectural philosophy is exactly what enterprises need in a cloud SBC provider. A session border controller that sits on redundant, globally distributed infrastructure means that even if one network path degrades, your Teams calls stay clear.

For organizations relying on Teams for mission-critical communications, the cloud SBC you choose is not just a technical decision — it is a business continuity decision.

3 options for adding an SBC to your Teams environment

Setting up VoIP can be a tangled affair, especially with an SBC in the equation. Issues surrounding security and quality of service (QoS) are paramount. There’s a wide array of factors to contemplate - from managing network traffic to constraining data rates. Opting to manage an SBC independently can be a costly and complex endeavor. Generally, there are three primary approaches to SBCs for Teams integration:

1. Do it yourself (Direct Routing only)

Implementing your own SBC offers maximum control and adaptability, but it comes with its own set of challenges and expenses. These challenges include:

• Ensuring robust security.
• Keeping everything up to date, from security to third-party compatibility to simple patching.
• Maintenance in case of issues, including changes as your network experiences changes—for example, a sudden increase in a particular type of traffic.

The challenges of the DIY route aren’t the only concerns that might make you look for other options. There are also potentially high costs to consider, including:

• SBC licensing costs can quickly reach thousands of dollars, which is significant for many companies.
• Staff time and salaries for maintenance or updates, as well as the opportunity costs of skilled workers spending time on technical administration as opposed to value-added development
• Security-related costs like audits, which might require more in-depth analysis for DIY setups.

Because of its complexities and higher costs, this route is only recommended for those with an experienced IT team or a knowledgeable partner in this realm. It's worth noting that a handful of Direct Routing providers like Telnyx offer an integrated, certified SBC, which means you don’t need to worry about costs or maintenance.

2. Microsoft's calling plans for Teams (an easy option)

For those looking to avoid complications, Microsoft’s Teams Phone plans offer a straightforward solution. Microsoft takes the reins, managing everything from the SBC to numbers and more.

The downside is that your telephony solutions are entirely dependent on Microsoft. This arrangement leaves you beholden to them for commercial and technical IT strategy. Other challenges to the Teams Calling plan approach include:

• Microsoft isn’t a specialist telephony vendor or registered carrier. While Microsoft is a large and diverse company with many interests, it’s not a registered carrier (as of February 2024). So considerations like phone number inventory involve third parties.
• Technical choices are strictly limited to whatever is permitted in a Teams Calling plan. Companies with complex requirements such as analog phones may find Teams Calling unfeasible for their configuration limitations.
• Available calling rates and bundles may feel limiting, especially for businesses with very niche or custom requirements, such as shared calling.

3. Managed SBC via Operator Connect or (in some cases) Direct Routing

Compared to the DIY and Teams Phone plan options, Operator Connect (OC) is an in-between approach to SBCs. With Operator Connect, you delegate the management of your telephony to experts in networking who are pre-vetted by Microsoft. However, you still maintain a level of independence and control over your connectivity technology.

To qualify as an OC provider, carriers undergo stringent checks and verifications, making it an excellent option for companies aiming to maintain autonomy from Microsoft, benefit from carrier-specific advantages, and avoid heavy investment in setup and maintenance.

While handing the stress of deploying an SBC over to an expert might be appealing, even managed SBCs have their challenges, including:

• As of February 2024, there are over 80 vendors on the Operator Connect Portal. The overwhelming amount of choice can make differentiating between vendors a tough call.
• Many vendors only support one country, which can be limiting for multinational businesses, which might then need to use multiple vendors with slightly different setups.
• While many vendors offer competitive per-minute calling rates and bundles, customers often overlook providers’ licensing models. Many vendors charge per seat, which can be expensive compared to those charging per phone number—a great option for setups with Shared Calling.

Every organization’s choice will depend on both its existing setup and future telephony needs. Since SBCs are required for Teams connections, choosing the right approach can mean the difference between simplifying your connectivity stack or making it even more complex.

Partner with Telnyx to make SBCs stress-free

If you’re already thinking of adding an SBC to your Teams setup, you’re probably aiming to reduce the complexities and concerns of communications and connectivity for your business. Choosing a provider that makes your SBC deployment simple and easy is crucial to avoid additional headaches and stress.

Telnyx supports two of the aforementioned pathways for SBC deployment: Direct Routing and Operator Connect. Our proprietary SBC technology balances control and customizability with cost. Choosing a Telnyx Teams integration gives you flexibility over commercial and technical choices you wouldn’t have with a Teams Calling Plan. As for costs, SBCs are integrated into our standard plans, so there’s no additional cost for deployment. That also means you can leave the maintenance effort and costs to our networking experts.

Telnyx operates a Tier 1 voice network with global coverage, which garners trust from industry giants such as Cisco and Avaya for real-time communication. This network is built on a robust multi-cloud infrastructure that can fail over in the event of an outage. This greater resiliency and reliability makes Telnyx an ideal choice for organizations that want to maintain connectivity while competitors are offline.

Frequently asked questions

What is a Session Border Controller (SBC) and why do I need one?

A Session Border Controller (SBC) is a device or software that manages and secures VoIP traffic at the border between different networks. It handles SIP signaling normalization, media transcoding, security (preventing toll fraud and DoS attacks), and interoperability between incompatible systems. Without an SBC, connecting a PSTN carrier to platforms like Microsoft Teams or Cisco is unreliable and insecure.

What is a cloud SBC?

A cloud SBC (cloud-based Session Border Controller) performs all traditional SBC functions in software hosted in the cloud rather than on-premises hardware. Cloud SBCs offer faster deployment, elastic scaling, no hardware maintenance, and geographic redundancy — making them ideal for enterprises with distributed workforces or hybrid environments.

How does an SBC work with Microsoft Teams Direct Routing?

Microsoft Teams doesn't support PSTN calls natively without an SBC. Direct Routing uses a certified SBC to connect Teams to a PSTN carrier, enabling users to make and receive real phone calls from within Teams. The SBC sits between the Teams Phone System and your carrier, handling SIP registration, call routing, and media relay.

What makes an SBC "Microsoft-certified"?

Microsoft certifies SBCs that pass interoperability testing with Teams Direct Routing. Certified SBCs are listed on Microsoft's approved vendor list and guarantee compatibility with Teams Phone System. Using a non-certified SBC risks call quality issues, failed connections, or voided Microsoft support agreements.

What's the difference between an SBC and a VoIP gateway?

A VoIP gateway converts analog or digital PSTN lines (POTS, ISDN) to VoIP. An SBC manages and secures IP-to-IP SIP traffic between two networks. Gateways are typically used to bridge legacy hardware; SBCs are used to interconnect modern VoIP systems, carriers, and cloud platforms with security and normalization capabilities.

How many sessions can a cloud SBC handle?

Cloud SBCs scale dynamically — you're not limited by hardware. Enterprise cloud SBC solutions handle hundreds to tens of thousands of simultaneous sessions. Providers offer tiered licensing based on concurrent call capacity, so you only pay for what you need and can scale up instantly during peak periods.

Can I replace my on-premises SBC with a cloud SBC?

Yes. Cloud SBCs have reached feature parity with on-premises hardware. The migration process typically involves pointing your SIP trunk registrations to the cloud SBC, configuring call routing rules, and testing. Cloud SBCs support all standard SBC functions: SIP normalization, DTMF handling, media transcoding, call recording integration, and security filtering.