Decoding DoT’s OSP Guidelines: PSTN, VoIP, and Microsoft Teams Direct Routing Explained

Balu Ilag | July 30th 2025

30 Jul Decoding DoT’s OSP Guidelines: PSTN, VoIP, and Microsoft Teams Direct Routing Explained

Posted at 21:50h in Microsoft Teams, Telecom by
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Understanding Indian DoT Regulations for PSTN Calling and VoIP in OSP Scenarios

India’s telecom regulatory landscape for Other Service Providers (OSPs)—including BPOs, KPOs, and call centers—has evolved significantly with the 2020 and 2021 updates (COVID Time) from the Department of Telecommunications (DoT). These changes simplify compliance, encourage remote working, and allow infrastructure flexibility, while continuing to enforce restrictions against toll bypass and ensure lawful usage of telecom resources.

This guide covers:

  • DoT vs TRAI: Understanding Their Roles in Indian Telecom
  • What OSP means and why it’s regulated
  • PSTN and VoIP usage rules
  • Toll bypass restrictions
  • Key changes in DoT 2020 & 2021 guidelines
  • Extending OSP license to new offices post-acquisition
  • Microsoft Teams Direct Routing compliance scenarios 

DoT vs TRAI:

Many people often confuse the roles of the Department of Telecommunications (DoT) and the Telecom Regulatory Authority of India (TRAI), but both serve distinct purposes in India’s telecom ecosystem. The DoT is a government department under the Ministry of Communications that creates telecom policies, issues licenses (such as for internet service providers or unified telecom licenses), allocates spectrum, and ensures compliance with security and regulatory frameworks, including the OSP guidelines. In contrast, TRAI is an independent regulatory body established to oversee fair competition, protect consumer interests, and regulate tariffs, interconnection charges, and quality of service among telecom operators. Simply put, the DoT functions as the policy maker and licensor, while TRAI acts as the referee ensuring that the market operates fairly and customers receive quality service.

What is OSP?

An Other Service Provider (OSP) is any entity using telecom resources for delivering application services such as call centers, tele-banking, tele-education, e-commerce support, or IT-enabled services. Examples include:

  • BPO/KPO companies
  • Inbound and outbound call centers
  • Tele-medicine or tele-trading services

Purpose of Regulation: DoT regulates OSPs to:

  • Prevent toll bypass (illegal routing of international calls over domestic networks)
  • Ensure lawful interception and national security
  • Promote responsible use of telecom infrastructure

PSTN and VoIP Rules in OSP Scenarios

PSTN Usage

  • Domestic and international PSTN traffic cannot interconnect.
  • PSTN resources must be obtained from licensed Indian Telecom Service Providers (TSPs).

VoIP Usage

  • VoIP is permitted but must comply with routing rules:
    • International VoIP must terminate via licensed International Long Distance (ILD) gateways.
    • No routing international calls through domestic PSTN.

Toll Bypass Restriction

Toll bypass refers to avoiding normal call charges by routing calls through private networks instead of using the public telephone network (PSTN). In India, regulations prohibit mixing enterprise VoIP calls directly with the PSTN in a way that skips the licensed telecom provider’s routes and charges.

Toll bypass occurs when international calls are routed through domestic PSTN networks to avoid international termination charges. This remains strictly prohibited.

Why do these rules exist? These rules ensure that all calls using the public telephone network go through authorized telecom providers. This prevents revenue loss for telecom operators and keeps the public network secure and reliable.

Example: If a U.S. customer call is terminated on Indian domestic lines without going through the ILD gateway, it constitutes toll bypass and is a regulatory violation.

Key Changes in 2020 & 2021 DoT Guidelines

2020 Guidelines (Nov 5, 2020)

  • OSP registration made voluntary
  • Shared infrastructure allowed (no need to segregate domestic/international)
  • Work-from-home agents permitted without special approval
  • Bank guarantees removed

2021 Clarification (June 23, 2021)

  • Confirmed centralized/shared EPABX allowed
  • Reiterated toll bypass prohibition
  • Compliance responsibility remains with OSP even without registration

Extending OSP License to New Offices Post-Acquisition

When a company with an OSP setup is acquired or expands to new locations:

  • Registration is not mandatory post-2020; compliance is sufficient.
  • Inform DoT of acquisition (good governance practice).
  • Add new centers without prior approval—ensure compliance with toll bypass prevention, PSTN segregation, and data security.
  • Maintain updated network diagrams and compliance documents.

Microsoft Teams Phone System Direct Routing Scenarios in OSP Context

Microsoft Teams Phone System, with Direct Routing, enables enterprises to connect Teams to the PSTN using Session Border Controllers (SBCs) and SIP trunks. In India, these deployments must follow DoT’s OSP regulations, especially for customer-facing scenarios.

  1. DoT Considerations for Teams Direct Routing
  • Toll Bypass Control: Ensure international PSTN calls do not route via domestic PSTN.
  • Telecom Resource Licensing: SIP trunks must come from DoT-licensed Indian TSPs.
  • VoIP Compliance: Teams-to-Teams VoIP is unrestricted, but PSTN breakout must align with DoT norms.
  • Remote Work Enablement: Teams supports DoT’s remote-agent model using secure VPN/MPLS.
  1. Scenarios

Internal Enterprise Telephony

  • Teams-to-Teams VoIP only; PSTN breakout via licensed SIP trunks.
  • OSP classification not required if strictly internal.

Call Center / BPO Operations

  • PSTN + VoIP usage qualifies as OSP.
  • Must prevent international/domestic PSTN interconnection.
  • Maintain call logs for lawful interception.
  1. Migrating Teams Direct Routing to New Offices

When relocating or expanding offices (e.g., post-M&A):

Step 1: Review classification (internal vs OSP) and compliance needs.
Step 2: Notify DoT of acquisition/new location (recommended).
Step 3: Procure new SIP trunks or extend existing ones from licensed TSP.
Step 4: Configure SBC for new location with PSTN segregation.
Step 5: Update network diagrams, call routing, and compliance documents.
Step 6: Migrate Teams users gradually to avoid downtime.

 

Best Practices for Complying with DoT OSP Guidelines and PSTN/VoIP Usage

  1. Use Licensed Telecom Providers Only: Always procure PSTN lines or SIP trunks from DoT-licensed Indian telecom service providers to ensure legal compliance.

  2. Prevent Toll Bypass: Maintain strict routing controls to avoid mixing international and domestic PSTN traffic. Configure SBCs (Session Border Controllers) to enforce separate routes.

  3. Document Your Architecture: Keep updated network diagrams showing PSTN, VoIP, and Teams Direct Routing configurations—especially during mergers, acquisitions, or office relocations.

  4. Follow Remote Work Compliance: When enabling work-from-home agents in Teams, use secure VPN/MPLS connections and ensure call recording/logging remains intact for lawful interception.

  5. Notify DoT of Significant Changes: While OSP registration is now voluntary, informing DoT about mergers, acquisitions, or major office relocations demonstrates good governance.

  6. Regular Compliance Audits: Conduct periodic internal audits to verify call routing, infrastructure sharing, and adherence to DoT’s security and privacy requirements.

  7. Plan Gradual Migration for New Offices: For Teams Direct Routing, migrate users in phases to minimize downtime and validate PSTN routing at each stage.

  8. Monitor Quality and Security Continuously: Use Microsoft Teams Admin Center or third-party monitoring tools to track call quality, detect anomalies, and ensure security controls remain effective.

Disclaimer

The information in this blog reflects personal interpretation of DoT guidelines and is for informational purposes only. For official guidance, M&A consultation, or help with compliance documentation and DoT communications, please contact me. 

References:

 

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