MICROSOFT TEAMS™ • VOICE • DIRECT ROUTING • SBC

Microsoft Teams Call Flow Explained: Signaling, Media, Direct Routing & SBC

A practical guide to understanding what happens when a Teams call starts, how SIP signaling and media move, and where Direct Routing, SBCs and the PSTN fit in.

Updated: August 24, 2026 • Reading time: 12–15 minutes

⚡ Short answer: A Microsoft Teams Call Flow has two important layers: signaling, which establishes and controls the call, and media, which carries the actual audio. In Direct Routing, signaling uses Microsoft’s SIP infrastructure while the media path depends on the deployment, including whether media bypass is used. Microsoft’s official call-flow documentation covers multiple scenarios.

Microsoft Teams Call Flow can look complicated because one voice call may involve the Teams client, Microsoft 365 services, voice-routing policies, a Session Border Controller (SBC), a PSTN carrier and several network paths. The easiest way to understand it is to follow the call in stages rather than trying to memorize every SIP message.

This SkypeExchange4U™ guide explains internal Teams calling, outbound and inbound PSTN calls, Direct Routing, media bypass, SIP flow, troubleshooting, PowerShell checks and common failure points in practical language.

🎯 Remember: Signaling tells the systems what to do. Media carries the conversation. A call can have successful signaling and still have a broken media path.

🚀 Quick Answer: How Does a Teams Call Flow Work?

Teams User → Teams Phone / Cloud → Voice Routing → SBC → PSTN / Carrier

For a simplified outbound Direct Routing call, Teams applies the relevant voice-routing configuration, signaling reaches the Direct Routing service and the organization’s SBC, and the SBC routes the call toward the PSTN carrier. The media path depends on the architecture.

🧰 What You’ll Need

Teams admin center access
Knowledge of the organization’s voice-routing design
Direct Routing/SBC access when required
Teams PowerShell for targeted checks
Caller and destination information
Exact call timestamp and timezone
Network, firewall and VPN information
Call-quality/session telemetry
CQD access for wider quality patterns

Microsoft documents per-user call analytics for investigating devices, networks, connectivity and call quality, while the Teams admin center provides session-level details.

1️⃣ Step 1 — Identify the Call Type

Call type	Main flow to understand
Teams → Teams	Teams service, client signaling and media
Teams → PSTN	Voice routing → Direct Routing → SBC → carrier
PSTN → Teams	Carrier → SBC → Direct Routing → Teams
Remote Direct Routing	Internet path and supported media design
Internal Direct Routing	SBC signaling plus selected media path

2️⃣ Step 2 — Understand Signaling vs. Media

📨 Signaling

Starts and controls the call
Exchanges session information
Handles call-state changes
Uses SIP in Direct Routing scenarios

🎙️ Media

Carries voice
Is sensitive to network conditions
Can follow different paths
Can fail even when signaling succeeds

Microsoft states that SIP traffic flows through the SIP proxy, while media behavior changes depending on whether Direct Routing media bypass is enabled.

3️⃣ Step 3 — Follow an Outbound PSTN Call

Teams User
↓
Teams Phone / Microsoft 365
↓
Voice Routing
↓
Direct Routing / Microsoft SIP Infrastructure
↓
Organization SBC
↓
PSTN Carrier
↓
External Number

The user selects an external number.
Teams processes the call and applies the user’s applicable voice-routing configuration.
Direct Routing signaling reaches the Microsoft SIP infrastructure and SBC path.
The SBC applies routing, normalization and interoperability rules.
The SBC sends the call toward the carrier or connected telephony system.
The destination network establishes the call.
Media follows the supported architecture.

The exact flow varies by topology, media bypass, user location and other deployment factors. Microsoft’s official call-flow documentation provides separate diagrams for several scenarios.

4️⃣ Step 4 — Follow an Inbound PSTN Call

External Number → PSTN Carrier → SBC → Direct Routing → Teams → User

The carrier delivers the call to the SBC. The SBC applies inbound routing and number handling, then the Direct Routing service delivers signaling toward the Teams user. Number normalization, SBC routing and media-bypass behavior can affect the final path. Microsoft documents dedicated inbound Direct Routing call-flow scenarios.

5️⃣ Step 5 — Understand Media Bypass

Media bypass is one of the most misunderstood parts of a Microsoft Teams call flow. In supported Direct Routing designs, media can use a shorter path between the Teams endpoint and SBC instead of the non-bypass media path. Microsoft’s guidance says media bypass can reduce unnecessary media hops, while signaling still uses the Microsoft cloud/SIP infrastructure.

💡 Important: Media bypass does not mean all signaling bypasses Microsoft. Signaling continues through the Microsoft SIP infrastructure while media can use the supported bypass path.

Potential benefits

Shorter media path in supported designs
Fewer unnecessary media hops
Useful for carefully designed enterprise networks

Trade-offs

More firewall/network planning
SBC compatibility requirements
ICE/ICE Lite considerations
More variables during troubleshooting

Microsoft documents supported SBC, port and media requirements for media bypass.

6️⃣ Step 6 — Internal vs. Remote Users

User location	Investigate
Corporate network	LAN, SBC reachability, firewall and media-bypass conditions
Home / Internet	Internet, VPN, NAT, firewall and supported media routing
Branch office	Site routing, local SBC, WAN and media optimization

Microsoft’s documented call flows distinguish internal and remote users and show different media behavior for bypass and non-bypass scenarios.

💻 Commands

Use PowerShell to confirm configuration and architecture. Avoid making production changes during diagnosis without a controlled change plan.

Get-CsOnlineUser -Identity user@contoso.com |
Format-List DisplayName,UserPrincipalName,LineURI,TeamsUpgradeEffectiveMode

Get-CsOnlinePSTNGateway |
Format-List Identity,Fqdn,SipSignalingPort,MaxConcurrentSessions,Enabled,MediaBypass

Get-CsOnlinePSTNGateway -Identity "sbc1.contoso.com" |
Format-List Identity,Fqdn,SipSignalingPort,MaxConcurrentSessions,Enabled,MediaBypass

Test-NetConnection sbc1.contoso.com -Port 5061

⚠️ Important: A successful TCP test does not prove SIP signaling, certificates, routing or media are healthy. It answers only a narrow connectivity question.

🧪 Example — Call Connects but There Is No Audio

Scenario: The Teams user calls an external number. The remote party answers, but neither side hears audio.

Wrong approach

Restart Teams → reinstall headset → change voice policy → restart SBC.

Better approach

Confirm signaling established.
Determine whether the media problem is one-way or two-way.
Review the affected session’s quality information.
Determine whether one user or many are affected.
Check VPN versus non-VPN behavior.
Identify the expected media path.
Check firewall/NAT and SBC media interfaces.
Compare with a known-good call.

🎯 Lesson: If signaling works but media does not, changing voice-routing policy is usually not the first place to look.

🚨 Common Errors

Symptom	Likely layer	First check
Call never connects	Routing / signaling	Policy, route, gateway and SIP flow
No audio	Media	Media path, firewall, NAT, VPN, SBC
One-way audio	Media	Direction of missing media
Outbound PSTN fails	Routing / SBC / carrier	Voice route and SBC trace
Inbound PSTN fails	Carrier / SBC / routing	Inbound number and SBC routing
Robotic audio	Network / media	Packet loss, jitter, latency

📨 SIP Call Flow: What Engineers Look At

Microsoft SIP Proxy
│
│ INVITE
↓
Organization SBC
│
│ INVITE / routing
↓
PSTN / Carrier
│
│ response / progress
↓
Call establishment

Microsoft provides a SIP call-flow view for Direct Routing in the Teams admin center. It presents SIP messages as a chronological ladder and allows administrators to inspect protocol details for individual events. Microsoft notes that SIP call data can take time to appear in reporting.

🔧 Troubleshooting

User cannot make PSTN calls

Check Teams Phone capability/licensing.
Check number assignment.
Check voice-routing policy.
Check PSTN usage and route selection.
Check Direct Routing gateway.
Check SBC SIP signaling.
Check carrier routing and response.

Inbound calls do not ring

Confirm carrier delivery to the SBC.
Inspect inbound SIP.
Verify called-number format and normalization.
Check SBC inbound routing.
Verify Teams number assignment.

Call quality is poor

Determine whether one user or a site is affected.
Review session quality information.
Check packet loss, jitter and latency indicators.
Compare Wi-Fi, wired and VPN behavior.
Identify the actual media path.
Check firewall/NAT and SBC media configuration.
Use CQD for wider patterns.

Microsoft’s call analytics guidance explains that per-user information includes device, network, connectivity and quality data useful for investigating individual calls.

⚖️ Pros and Cons

✅ Pros of Direct Routing

Integrates Teams with enterprise telephony
Provides SBC and carrier control
Supports complex voice architectures
Can support media optimization scenarios
Useful for existing PSTN investments

❌ Cons / Trade-offs

More components mean more failure points
SBC and carrier expertise may be required
Certificates and firewall design add work
Media bypass needs additional planning
Carrier behavior can complicate diagnosis

⏱️ 30-Second Microsoft Teams Call-Flow Checklist

When a call fails, check these in order:

What type of call is it? Teams-to-Teams, outbound PSTN, inbound PSTN, transfer or conference?
Did signaling start? If not, investigate user configuration, routing or connectivity.
What was the last successful hop? Teams → Microsoft cloud → SBC → carrier?
Did media establish? If signaling succeeds but audio fails, investigate the media path.
Is the issue isolated or widespread? One user points toward endpoint/configuration; many users can indicate site, SBC, carrier or service impact.
What evidence do you have? Use Call Analytics, SIP ladder data, SBC logs and the exact call timestamp.

🏆 Best Practices

Document signaling and media separately.
Draw the call flow. A diagram can expose missing dependencies.
Document every SBC and carrier hop.
Record number-normalization rules.
Keep a known-good test user.
Capture exact timestamps.
Use Teams admin center session evidence.
Use SIP ladder data for Direct Routing signaling.
Use CQD for wider quality patterns.
Change one production variable at a time.
Validate firewall and media requirements before bypass changes.
Keep documentation aligned with current Microsoft guidance.

🌳 Simple Call-Flow Troubleshooting Decision Tree

Did the call start? → No: investigate configuration, routing and signaling.

Did it reach the expected next hop? → No: investigate the last successful component.

Did media establish? → No: follow media path, firewall/NAT, VPN and SBC media configuration.

Is media quality poor? → Investigate packet loss, jitter, latency, Wi-Fi, WAN and congestion.

🔗 Related Articles

How an SBC Works with Microsoft Teams
SBC architecture, Direct Routing, SIP, media and routing.
Microsoft Teams Voice Troubleshooting Guide
Diagnose call failures, poor audio, Direct Routing and SBC problems.
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Explore AI and technology coverage from SkypeExchange4U™.

🔍 Evidence Before Configuration Changes

For production troubleshooting, avoid changing several settings at once. First capture the caller, destination, timestamp, call direction, affected site, SBC, SIP response and media symptom. Then make the smallest controlled change that tests your hypothesis.

Microsoft’s current Direct Routing guidance also provides built-in diagnostic tests in the Microsoft 365 admin center and a SIP call-flow view for examining SIP requests, responses and SDP exchanged between the Teams proxy and SBC.

❓ Microsoft Teams Call Flow FAQs

What is a Microsoft Teams call flow?

It is the sequence of signaling, routing and media interactions that occur when a Teams user makes or receives a call. The exact path depends on the architecture.

How does Teams Direct Routing call flow work?

Teams applies voice routing, signaling reaches the Direct Routing service and organization’s SBC, and the SBC routes the call toward the PSTN carrier. Media follows the supported architecture.

What is the difference between signaling and media?

Signaling controls call setup and session information. Media carries the actual audio. Successful signaling does not guarantee healthy media.

What does an SBC do in Teams Direct Routing?

The SBC provides the controlled voice boundary between Direct Routing and the organization’s PSTN or telephony environment, handling routing and interoperability functions.

What is media bypass?

Media bypass is a Direct Routing design in which supported media can travel directly between the Teams endpoint and SBC instead of following the non-bypass media path. Signaling still uses Microsoft SIP infrastructure.

How do I troubleshoot a Teams call flow?

Identify the call type, separate signaling from media, determine where the call stopped, inspect session and SIP evidence, then follow the network and SBC path to the last successful component.

How quickly does SIP call-flow data appear in Teams admin center?

Microsoft says to allow up to 30 minutes for SIP call-flow data to be processed and uploaded for reporting, and call records older than 30 days are not available in the SIP call-flow view.

Where can I see the Direct Routing SIP call flow?

Microsoft provides a SIP call-flow view in the Teams admin center for Direct Routing. It presents SIP messages as a chronological ladder.

🎯 The Bottom Line

The easiest way to understand Microsoft Teams Call Flow is to follow the call in layers.

Identify the call type. Separate signaling from media. Follow routing, SBC and carrier paths. Then identify exactly where the call stopped or where quality degraded.

Don’t ask only “Why did the call fail?” Ask “Where did the call stop, and what proves it?”

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SkypeExchange4U™ publishes practical guides covering Microsoft Teams, enterprise voice, AI, cloud communications and emerging technology.

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Editorial & trademark note: Microsoft Teams, Microsoft, Teams Phone and other Microsoft product names are trademarks of Microsoft Corporation. SkypeExchange4U™ is an independent publication and is not affiliated with or endorsed by Microsoft. Verify current Microsoft documentation before making production changes.

📚 Sources & Further Reading

Technical details in this guide are based primarily on Microsoft Learn documentation. Always verify production changes against the current Microsoft and SBC-vendor documentation.

Freshness note: Microsoft Teams voice features, supported SBC versions, network requirements and troubleshooting tools can change. This guide is an educational reference, not a substitute for current Microsoft or SBC-vendor deployment documentation.