Microsoft Teams Direct Routing allows organizations to connect Microsoft Teams with the Public Switched Telephone Network (PSTN) through a Session Border Controller (SBC) and a certified VoIP/SIP provider.
This guide explains the complete Microsoft Teams Direct Routing architecture, outgoing and incoming call flows, SBC requirements, SIP connectivity, certificates, firewall configuration, voice routing and important technical considerations.
What Is Microsoft Teams Direct Routing?
Microsoft Teams Direct Routing is a telephony connectivity solution that allows Microsoft Teams users to make and receive external telephone calls by connecting Teams to the PSTN through an SBC.
Instead of relying only on Microsoft’s own calling connectivity, an organization can use a compatible VoIP provider or SIP trunk provider and connect that service to Microsoft Teams using an approved Session Border Controller.
The basic communication path is:
Microsoft Teams → SBC → VoIP/SIP Provider → PSTN
For incoming calls, the direction is reversed:
PSTN → VoIP/SIP Provider → SBC → Microsoft Teams
Microsoft Teams Direct Routing Architecture
The architecture shown in the diagram contains four major components:
- Microsoft Teams
- Session Border Controller (SBC)
- VoIP Provider / SIP Trunk
- PSTN
1. Microsoft Teams
Microsoft Teams provides the user-facing communication experience. Users can make and receive calls using the Teams desktop application, web application, mobile application or supported Teams devices.
To use Direct Routing, the organization needs the appropriate Microsoft 365 and Teams telephony configuration, including users, phone numbers and voice-routing policies.
2. Session Border Controller (SBC)
The Session Border Controller is a critical component in a Direct Routing deployment. It sits between Microsoft Teams and the external SIP/VoIP provider.
The SBC provides functions such as:
- SIP signaling control
- Media handling
- Interoperability between systems
- Call routing
- Security controls
- SIP normalization and manipulation where required
- Connectivity between Teams and the SIP provider
3. VoIP Provider / SIP Trunk
The VoIP provider supplies the SIP connectivity required to reach external telephone networks. Depending on the provider, the service may include telephone numbers, SIP trunks, routing, PSTN termination and incoming call delivery.
4. PSTN
PSTN stands for Public Switched Telephone Network. It represents the traditional telephone network used to connect calls to mobile phones, landlines and other external telephone numbers.
Microsoft Teams Direct Routing Call Flow
Direct Routing has two primary call scenarios:
- Outgoing calls: Microsoft Teams user calls an external telephone number.
- Incoming calls: An external telephone number calls a Microsoft Teams user.
Outgoing Call Flow: Microsoft Teams to PSTN
The typical outgoing call path is:
Teams User → SBC → VoIP Provider → PSTN → External Number
- Teams user starts the call. The user enters an external telephone number in Microsoft Teams.
- Microsoft Teams processes the call. Teams applies the organization’s configured voice-routing rules and policies.
- Call is sent to the SBC. The SBC receives the signaling from Microsoft Teams.
- SBC routes the call to the VoIP provider. The SBC sends the SIP signaling toward the configured SIP trunk/provider.
- VoIP provider connects to the PSTN. The provider routes the call toward the destination telephone network.
- The destination phone rings. The external mobile phone, landline or other PSTN destination receives the call.
Incoming Call Flow: PSTN to Microsoft Teams
The incoming call follows the reverse path:
PSTN → VoIP Provider → SBC → Microsoft Teams → Teams User
- External caller dials the business number.
- PSTN sends the call to the VoIP provider.
- VoIP provider sends the SIP call to the SBC.
- SBC processes and routes the call. The SBC determines where the call should be delivered.
- Microsoft Teams receives the call.
- The assigned Teams user receives the incoming call.
What Is Required for Microsoft Teams Direct Routing?
A successful Direct Routing deployment requires several components to be prepared before configuration.
Microsoft 365 and Microsoft Teams Requirements
- Microsoft 365 tenant
- Microsoft Teams
- Appropriate Teams Phone licensing
- Users enabled for enterprise voice/telephony
- Phone numbers assigned to users where required
- Voice routing configuration
- Appropriate dial plans and normalization rules
SBC Requirements
- Supported or certified SBC platform
- Publicly resolvable SBC FQDN
- Valid TLS certificate
- Correct SIP configuration
- Microsoft Teams Direct Routing configuration
- SIP trunk configuration toward the provider
- Correct media and signaling configuration
- Voice routing configuration
VoIP Provider Requirements
- SIP trunk or compatible VoIP service
- Telephone numbers
- SIP authentication or IP-based authorization, depending on the provider
- Inbound and outbound call routing
- Supported codecs
- PSTN connectivity
- Emergency calling support where applicable
Network and Firewall Requirements
Network configuration is one of the most important parts of a Microsoft Teams Direct Routing deployment. Incorrect firewall or NAT configuration can result in failed calls, one-way audio or no audio.
Before deployment, review:
- Public IP requirements
- DNS configuration
- Firewall rules
- SIP signaling connectivity
- Media/RTP connectivity
- NAT configuration
- Required Microsoft 365 endpoints
- VoIP provider network requirements
Always use the current Microsoft and SBC/provider documentation for exact ports, IP ranges and supported configurations because these requirements can change.
SBC Certificate Requirements
A valid certificate is essential for secure SIP/TLS communication between Microsoft Teams and the SBC.
The SBC’s FQDN and certificate configuration must meet the requirements of the Direct Routing deployment. Certificate configuration should be checked carefully before attempting production calls.
Important certificate considerations include:
- Correct SBC FQDN
- Valid certificate chain
- Certificate expiration date
- Correct subject/SAN configuration
- Trusted certificate authority
- Proper certificate installation on the SBC
- TLS configuration
SIP Trunk and VoIP Provider Configuration
The SBC needs a SIP connection to the selected VoIP provider. The exact configuration depends on the provider and SBC manufacturer.
Typical SIP trunk configuration can include:
- SIP server or proxy address
- SIP transport
- Authentication method
- Caller ID configuration
- Number formatting
- Inbound routing
- Outbound routing
- Codec configuration
- DTMF configuration
- Media configuration
Voice Routing in Microsoft Teams
Voice routing determines how Microsoft Teams handles external telephone calls. A typical deployment may contain:
- Dial plans
- Normalization rules
- Voice routes
- Voice routing policies
- PSTN usages
- SBC configuration
Correct number normalization is especially important. The telephone number presented by the user may need to be converted into the format expected by the SBC or VoIP provider.
Security Considerations for Direct Routing
Because Direct Routing connects enterprise communications with external telephone networks, security should be considered throughout the deployment.
- Use secure SIP/TLS communication where required.
- Protect the SBC from unauthorized access.
- Restrict unnecessary firewall access.
- Monitor SIP authentication and connection attempts.
- Review outbound calling permissions.
- Monitor unusual call activity.
- Keep SBC software and firmware updated.
- Use appropriate certificate management practices.
Common Microsoft Teams Direct Routing Problems
During deployment, administrators may encounter several common call-quality or connectivity problems.
1. Outgoing Calls Are Failing
Check:
- Teams Phone configuration
- User voice routing policy
- Dial plan and normalization rules
- SBC route configuration
- SIP trunk status
- VoIP provider routing
- Destination number format
2. Incoming Calls Are Not Reaching Teams
Check:
- Assigned phone number
- Inbound SBC routing
- VoIP provider routing
- Number normalization
- Teams user configuration
- Firewall and SIP connectivity
3. One-Way Audio
One-way audio is frequently associated with media connectivity, firewall, NAT or RTP-related configuration. Check the media path between the relevant endpoints and verify the SBC’s network configuration.
4. No Audio
If a call connects but there is no audio, investigate RTP/media connectivity, firewall rules, NAT behavior, codec compatibility and SBC media configuration.
Microsoft Teams Direct Routing Architecture Diagram
Microsoft Teams Direct Routing: Simple Architecture
Microsoft Teams
↓
Session Border Controller (SBC)
↓
SIP Trunk / VoIP Provider
↓
PSTN
↓
Mobile / Landline / External Telephone Network
Benefits of Microsoft Teams Direct Routing
Direct Routing can be useful for organizations that need to integrate Microsoft Teams with an existing telephony environment or a third-party SIP/VoIP provider.
- Connect Microsoft Teams with external telephone networks.
- Use compatible third-party SIP/VoIP services.
- Integrate Teams with enterprise telephony infrastructure.
- Support inbound and outbound PSTN calling.
- Use an SBC for SIP and telephony interoperability.
- Provide centralized call-routing control.
- Support enterprise voice architectures with appropriate design.
Microsoft Teams Direct Routing vs Traditional VoIP
| Feature | Microsoft Teams Direct Routing | Traditional VoIP/SIP |
|---|---|---|
| User application | Microsoft Teams | VoIP phone, softphone or PBX |
| SBC | Typically used for Direct Routing | Depends on architecture |
| PSTN connectivity | Through configured telephony connectivity | Through SIP/VoIP provider |
| Call routing | Teams + SBC + provider routing | PBX/SBC/provider routing |
| Collaboration | Integrated with Microsoft Teams | Depends on platform |
Microsoft Teams Direct Routing Deployment Checklist
Use this checklist before moving the solution into production:
- ☐ Microsoft 365 tenant configured
- ☐ Teams Phone licensing checked
- ☐ Users configured
- ☐ Telephone numbers assigned
- ☐ Certified/supported SBC selected
- ☐ SBC FQDN configured
- ☐ Public DNS configured
- ☐ Certificate installed and validated
- ☐ Firewall rules configured
- ☐ SIP trunk configured
- ☐ VoIP provider connectivity tested
- ☐ Dial plans configured
- ☐ Normalization rules tested
- ☐ Voice routes configured
- ☐ Voice routing policies configured
- ☐ Inbound calls tested
- ☐ Outbound calls tested
- ☐ Caller ID tested
- ☐ Audio quality tested
- ☐ Emergency calling requirements reviewed
- ☐ Monitoring and troubleshooting procedures documented
Frequently Asked Questions About Microsoft Teams Direct Routing
What is Microsoft Teams Direct Routing?
Microsoft Teams Direct Routing is a telephony integration method that connects Microsoft Teams to the PSTN through a compatible Session Border Controller and a supported telephony or SIP provider.
What is an SBC in Microsoft Teams Direct Routing?
SBC stands for Session Border Controller. It provides the connection and control point between Microsoft Teams and external SIP/VoIP infrastructure and can handle signaling, security, interoperability and call routing functions.
Can Microsoft Teams make normal mobile and landline calls?
Yes. With the appropriate Teams Phone and PSTN connectivity configuration, Teams users can make and receive calls to external telephone numbers through the configured telephony infrastructure.
Is an SBC required for Direct Routing?
Direct Routing uses a certified or supported SBC as the connection point between Microsoft Teams and the organization’s telephony infrastructure.
What is a SIP trunk?
A SIP trunk is a VoIP-based connection used to carry telephone calls between an organization’s telephony system and a service provider.
What is the difference between SIP and PSTN?
SIP is a signaling protocol commonly used for IP-based voice communications, while PSTN refers to the traditional public telephone network. SIP/VoIP infrastructure can provide connectivity between enterprise systems and the PSTN.
Why is a certificate required for Teams Direct Routing?
Secure communications between Microsoft Teams and the SBC rely on appropriate certificate and TLS configuration. The certificate must meet the requirements of the Direct Routing deployment.
Why is there one-way audio in Teams Direct Routing?
One-way audio can be caused by media connectivity problems, firewall rules, NAT, RTP configuration, codec issues or incorrect SBC configuration. The signaling path and media path should be investigated separately.
Can an existing VoIP provider be used with Microsoft Teams Direct Routing?
It depends on the provider, SBC and Microsoft Teams compatibility requirements. The provider must support the required SIP connectivity and the overall architecture must meet Microsoft’s current Direct Routing requirements.
What should be checked when outbound Teams calls fail?
Check the user’s Teams Phone configuration, voice routing policy, dial plan, normalization rules, SBC routing, SIP trunk connectivity, number format and VoIP provider configuration.
Conclusion
Microsoft Teams Direct Routing provides a way to connect Microsoft Teams users with external telephone networks using an SBC and compatible VoIP/SIP infrastructure. The basic architecture consists of Microsoft Teams, SBC, VoIP Provider and PSTN.
A successful deployment requires careful planning of Microsoft 365 licensing, Teams Phone, SBC configuration, certificates, DNS, firewall rules, SIP trunks, voice routing, number formatting, media connectivity and security.
Understanding both the outgoing call flow and the incoming call flow makes it significantly easier to design, configure and troubleshoot a Teams Direct Routing environment.
