MS-721: Which PSTN Connectivity Model Ensures Call Continuity During Internet Outages in Microsoft Teams?

Discover why Direct Routing is the optimal PSTN model for Microsoft Teams deployments with legacy PBX systems and intermittent internet connectivity.

Question

You have a Microsoft Teams Phone deployment that contains a branch office in Rochester. The Rochester branch contains a legacy PBX system.

You plan to migrate the PBX to Teams. The Rochester office has intermittent internet connectivity.

You need to recommend a PSTN model that ensures that users at the Rochester office can make calls regardless of internet connectivity.

Which PSTN connectivity model should you recommend?

Select only one answer.

A. Calling Plans
B. Direct Routing
C. Operator Connect
D. Teams Phone Mobile

Answer

To address the scenario of migrating a legacy PBX to Microsoft Teams in an office with unreliable internet, Direct Routing (Option B) is the recommended PSTN connectivity model.

B. Direct Routing

Explanation

The answer is Direct Routing. You need the support of an SBA to ensure that calling is always available, regardless of internet connectivity, and this is the only option that supports this.

Legacy PBX Integration

Direct Routing allows seamless interoperability with on-premises telephony infrastructure, such as legacy PBX systems. By connecting a supported Session Border Controller (SBC) to Teams, calls can route through the existing PBX during internet outages, ensuring continuity.

Local Failover Capability

With Direct Routing:

• Calls remain local if internet connectivity drops, leveraging the PBX’s PSTN lines.
• The SBC acts as a bridge between Teams and the PBX, enabling hybrid routing (e.g., internal calls via Teams, PSTN calls via the PBX).

Internet-Independent Resilience

Unlike cloud-only solutions (Calling Plans, Operator Connect, or Teams Phone Mobile), Direct Routing does not rely solely on internet connectivity. This is critical for offices with intermittent internet.

Why Other Options Fail

• Calling Plans (A): Requires stable internet for cloud-based PSTN routing. No fallback to local infrastructure.
• Operator Connect (C): Depends on third-party carriers via the cloud, which still requires internet access.
• Teams Phone Mobile (D): Relies on cellular networks and internet for Teams integration, making it unsuitable for connectivity gaps.

Key Considerations for Direct Routing

• Requires deploying an SBC (e.g., AudioCodes, Ribbon) to interface with the PBX.
• Configure voice routing policies to prioritize local PSTN trunks during outages.

By implementing Direct Routing, the Rochester office maintains call functionality regardless of internet stability, ensuring a smooth migration from the legacy PBX to Teams.

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