Microsoft Teams to PSTN Call Flow with Components and Diagram

Microsoft Teams Phone System allows users to make calls to traditional phone numbers (PSTN). Understanding the call flow helps in troubleshooting call issues, designing voice architecture, and managing enterprise telephony solutions.

Main Components Involved

• Microsoft Teams Client (PC, Mobile, Desk Phone)
• Teams Phone System (Cloud)
• Dial Plan and Voice Routing Policy
• Session Border Controller (SBC) – for Direct Routing
• PSTN Carrier / Telecom Provider
• Public Switched Telephone Network (PSTN)
• Called Party (Mobile or Landline)

Teams to PSTN Call Flow (Step-by-Step)

Step 1: User Dials Number

The user dials a PSTN number from the Microsoft Teams client. Example: +91XXXXXXXXXX.

Step 2: Policy and Dial Plan Processing

The Teams Phone System checks the dial plan to normalize the number and applies the voice routing policy and PSTN usage to determine the correct route.

Step 3: Call Sent to SBC (Direct Routing)

Teams Client
     |
     v
Microsoft Teams Phone System
     |
     v
Session Border Controller (SBC)

The SBC provides security using TLS and SRTP, performs SIP signaling conversion, and manages NAT traversal.

Step 4: SBC Sends Call to PSTN Carrier

SBC  --->  PSTN Carrier (SIP Trunk)

Step 5: PSTN Delivers Call to Destination

PSTN Network ---> Mobile or Landline User

Full Call Flow Diagram

Teams Client
     |
     v
Microsoft Teams Phone System
     |
     v
Voice Routing Policy + Dial Plan
     |
     v
Session Border Controller (SBC)
     |
     v
PSTN Carrier
     |
     v
PSTN Network
     |
     v
Called Party (Mobile/Landline)

High-Level Architecture Diagram

+----------------+
|  Teams Client  |
| (PC / Mobile)  |
+--------+-------+
         |
         v
+---------------------------+
| Microsoft Teams Cloud     |
|  (Phone System)           |
+-------------+-------------+
              |
              v
     +------------------+
     |   SBC (On-Prem)  |
     +--------+---------+
              |
              v
     +------------------+
     | PSTN Carrier     |
     +--------+---------+
              |
              v
        +-------------+
        |  PSTN       |
        |  Network    |
        +------+------+ 
               |
               v
        +-------------+
        | Called User |
        | (Mobile)    |
        +-------------+

Call Signaling vs Media Flow

(Call Signaling - SIP)
Teams Client ---> Teams Cloud ---> SBC ---> Carrier

(Media - RTP/SRTP)
Teams Client <==============================> SBC

Operator Connect Call Flow Diagram

+----------------+
| Teams Client   |
+--------+-------+
         |
         v
+---------------------------+
| Microsoft Teams Cloud     |
+-------------+-------------+
              |
              v
     +---------------------+
     | Operator Connect    |
     | Carrier             |
     +----------+----------+
                |
                v
          +------------+
          |   PSTN     |
          +------+-----+
                 |
                 v
          +------------+
          | Called User|
          +------------+

Protocols Used

• SIP – Call signaling
• TLS – Secure signaling
• RTP – Media transport
• SRTP – Encrypted media

Summary

The Microsoft Teams to PSTN call flow involves the Teams client sending a call to the Teams Phone System, which applies dial plans and routing policies. The call is routed either through an SBC (Direct Routing) or directly to an Operator Connect provider, and finally delivered through the PSTN network to the destination phone number.

Frequently Asked Questions (FAQ)

What is an SBC in Microsoft Teams Direct Routing?

An SBC is a gateway that connects Microsoft Teams to the PSTN carrier while providing security, protocol translation, and call control.

What is the difference between Direct Routing and Operator Connect?

Direct Routing requires an SBC on the customer side, while Operator Connect connects Teams directly to the carrier without needing customer SBC hardware.

Which protocol does Teams use for PSTN calls?

Microsoft Teams uses SIP for signaling and SRTP for encrypted voice media.

Why is understanding call flow important?

Understanding the call flow helps troubleshoot call failures, design voice architecture, and optimize call quality.