1. Teams Calling Hierarchy & Architecture

Teams Calling (Teams Phone) is a cloud-based private branch exchange (PABX) system that connects the Microsoft Teams client to the Public Switched Telephone Network (PSTN).

  • The PABX Service: Microsoft Phone System provides call control, auto-attendants, call queues, and voicemail.
  • Logical Layers:
    • The Client: Teams app (Desktop, Mobile, or IP Phone) where the user interacts with the dialer.
    • The Signal: SIP signaling traffic always travels to the nearest Microsoft 365 server “front door”.
    • The Media: Voice/Video traffic (SRTP) follows the most direct path between participants or to the PSTN gateway.
    • The Gateway: The bridge to the global telephone network.

2. PSTN Connectivity Models

Choosing a connectivity model is the most critical architectural decision for a voice deployment.

  • Microsoft Calling Plans: Microsoft acts as the carrier. Simple “all-in-the-cloud” setup with no infrastructure to manage, but can be more expensive per user.
  • Operator Connect: A “happy medium.” You choose a certified third-party carrier (e.g., Telstra, Vodafone) from within the Teams Admin Center. The operator manages the hardware (SBCs) while you manage the numbers in the cloud.
  • Direct Routing: The “Swiss Army Knife.” Connects Teams to any local SIP trunk or legacy PABX using a Session Border Controller (SBC).
    • Media Bypass: A Direct Routing feature allowing media to flow directly between the client and the SBC, bypassing the Microsoft cloud to reduce latency and improve quality.

3. Call Flow & Media Logic

Understanding the distinction between how a call is controlled (Signaling) and how it is heard (Media) is the key to solving most “one-way audio” or “robotic voice” issues.

  • Peer-to-Peer (P2P): For calls between two internal Teams users, media takes the shortest path. This is typically Direct IP-to-IP if users are on the same network, or via a Microsoft Transport Relay if firewalls block the direct path.
  • PSTN Flow (Signaling): The “command” data that tells the phone to ring always travels via the Microsoft Cloud: Client → Microsoft Teams → SBC/Carrier → PSTN.
  • PSTN Flow (Media): The actual voice packets follow one of two logic paths based on your Media Bypass configuration:
    • Option A: Media Bypass ON (The “Direct” Path): Audio flows directly from the Client → SBC/Gateway. This is the preferred “Gold Standard” as it minimizes latency by keeping the audio stream out of the Microsoft Cloud.
    • Option B: Media Bypass OFF (The “Cloud” Path): Audio flows Client → Microsoft Media Processor → SBC/Gateway. In this scenario, the Microsoft Cloud acts as a middleman. This is the required fallback when a client cannot establish a direct network connection to the SBC’s IP.
  • Quality Requirements: As per the VoIP Fundamentals article, for reliable voice quality, the network path must adhere to these strict thresholds:
    • Latency (RTT): < 200ms.
    • Jitter: < 30ms.
    • Packet Loss: < 1%.

4. Voice Routing Components

  • Dial Plans: A set of normalization rules that translate a user’s dialed numbers (e.g., 0412...) into E.164 format (+61412...).
  • Voice Routing Policies: Dictate which PSTN usage records (and therefore which SBCs/Routes) a user can access.
  • Emergency Routing: Defines how emergency calls (e.g., 000, 911) are handled and ensure location data is sent to the dispatcher.

5. Diagnostics & Troubleshooting

Understanding how to interpret telemetry data is the final step in resolving voice quality issues.

  • Real-time Call Health: During a call, users can check Settings > Call Health to see active packet loss and jitter stats.
  • Call Analytics (Per-User): Found in the Teams Admin Center under the user’s profile. Provides technical telemetry (device used, network type, signal strength) for every call in the last 30 days.
  • Call Quality Dashboard (CQD): An organization-wide dashboard for identifying systemic issues (e.g., “Why is audio quality poor specifically at the Sydney branch?”).
  • PSTN Usage Report: Found in the Teams Admin Center under Analytics & reports. This provides a granular record of all PSTN activity, including call duration, start/end times, and phone numbers for Calling Plans, Operator Connect, and Direct Routing. It is the primary tool for verifying if a call successfully reached the PSTN and for diagnosing billing or connectivity discrepancies.
  • Common Issue: One-Way Audio:
    • Typically caused by NAT/Firewall issues where the media stream is blocked in one direction.
    • In Direct Routing, often indicates the SBC is not sending media to the correct public IP of the Teams client or Media Processor.

6. Essential PowerShell Cmdlets (MicrosoftTeams Module)

  • Connectivity: Connect-MicrosoftTeams
  • Number Assignment (Modern):
    • Set-CsPhoneNumberAssignment -Identity user@domain.com -PhoneNumber "+123456789" -PhoneNumberType CallingPlan.
  • Troubleshooting User Config:
    • Get-CsOnlineUser -Identity user@domain.com | Select-Object EnterpriseVoiceEnabled, LineURI, VoiceRoutingPolicy.
  • Auto Attendants & Queues:
    • Get-CsAutoAttendant / Get-CsCallQueue.
  • Direct Routing SBC Check:
    • Get-CsOnlineDirectRoutingSBCStatus (Checks if your SBC is successfully talking to Microsoft).