Microsoft Teams Call Flow

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In Microsoft Teams, when using a Session Border Controller (SBC) and SIP trunks for Direct Routing, the call flow typically involves several key steps. Here’s an overview of how it works:

1. Call Origination

  • Internal Call: A user within the Microsoft Teams environment initiates a call from the Teams client (desktop, mobile, or web).
  • External Call: An external call is made from the PSTN (Public Switched Telephone Network) towards a Teams user.

2. Routing via Microsoft Teams

  • For internal calls, Microsoft Teams routes the call through its cloud infrastructure.
  • For external calls, the call is routed to the Microsoft Phone System in the cloud.

3. Session Border Controller (SBC)

  • The SBC is deployed on-premises or in the cloud by the organization.
  • The SBC securely connects the Microsoft Phone System to the PSTN via SIP trunks.
  • For outgoing calls: Teams sends the call to the SBC, which forwards it to the SIP trunk provider.
  • For incoming calls: The SBC receives the call from the SIP trunk and routes it to the Microsoft Phone System.

4. SIP Trunk

  • The SIP trunk serves as the communication line between the SBC and the PSTN.
  • Outgoing calls: The SIP trunk provider routes the call to the external phone number via the PSTN.
  • Incoming calls: Calls from the PSTN are routed via the SIP trunk to the SBC.

5. Call Termination

  • Internal Calls: The call is received by the other Teams user through the Microsoft Teams environment.
  • External Calls: The call is routed to the intended external phone number via the SIP trunk and PSTN.

6. Media Flow

  • The media (voice data) is typically transmitted directly between the Teams client and the SBC to minimize latency and optimize call quality. This can occur directly if the SBC is located on-premises or through the cloud if the SBC is hosted.
  • The SBC handles tasks such as codec translation, NAT traversal, and security enforcement to ensure the call quality and security.

7. Call Control

  • Call control signals (SIP messages) are exchanged between Microsoft Teams, the SBC, and the SIP trunk provider to manage the call setup, in-call features, and teardown.

Botton Line:

The call flow in Microsoft Teams using an SBC and SIP trunks is a seamless process that bridges the Teams cloud environment with traditional telephony systems, allowing for efficient communication between internal Teams users and external parties via the PSTN. The SBC plays a critical role in securing and managing the connection between Microsoft Teams and the SIP trunks.

I recommend reaching out to the engineering teams at Ubilynx.com or SWCTN.com for more information on this blog post.

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