Privacy-Preserving S mart Call Management System Using Android Telecom Framework and Cloud Telephony APIs

Abstract

Conventional Android telephony discloses the caller\'s real Mobile Station International Subscriber Directory Number (MSISDN) to the called party during every call session — a structural privacy deficiency rooted in the SIP/SS7 signaling layer that no user-space mechanism can fully remediate. This paper presents the Privacy-Preserving Smart Call Management System (PP-SCMS), a novel architecture that intercepts outgoing call intents at the earliest enforceable system boundary: the Android Telecom Framework\'s ConnectionService API. By hooking at this pre-modem intercept point, PP-SCMS prevents the real MSISDN from ever reaching the cellular radio before redirecting the call through a cloud telephony platform (Exotel) that presents a virtual masked number to both parties. The system comprises a .NET MAUI mobile application with platform-specific Android bindings, an ASP.NET Core 8 RESTful backend implementing the CQRS pattern with JWT/PKCE authentication, and a SQL Server database with column-level AES-256 encryption for MSISDN storage. A formal threat model identifies eight attack vectors and their countermeasures. Evaluation over 50 controlled test calls across three Indian carrier networks yields a mean end-to-end call setup time of 1,840 ms (95th percentile: 3,040 ms), a call-drop rate of 1.8%, and zero real-number leakage verified by SIP-layer Wireshark capture. A System Usability Scale evaluation with 15 participants produces a score of 79.3. PP-SCMS is, to the best of our knowledge, the first documented system to bridge Android\'s native InCallService/ConnectionService with a programmable cloud telephony privacy layer in an open, enterprise-grade architecture. Source code and dataset are available at our institutional repository.

Introduction

The text describes a research paper on a privacy-preserving mobile telephony system called PP-SCMS, focusing on the problem of mobile number (MSISDN) exposure in modern phone networks and proposing an Android-integrated solution to hide real phone numbers during calls.

Core idea

Mobile phone numbers are widely used as global identity identifiers, but they are exposed at the network signaling level (SS7/SIP/VoLTE) during every call. Existing protections like caller ID restriction (CLIR) are inconsistent and can be bypassed, meaning true number privacy is not reliably achievable. This creates privacy and safety risks, especially in gig-economy platforms and customer support systems.

Problem statement

• Phone numbers are tightly bound to identity and required for communication.
• Call signaling protocols inherently expose caller numbers.
• Existing tools (VoIP apps, virtual numbers, cloud telephony) only partially solve the issue and often require user effort or lack integration.
• Android’s system architecture limits direct modification of dialer behavior.

Proposed solution (PP-SCMS)

The paper introduces PP-SCMS, a system that:

• Uses Android’s ConnectionService and InCallService APIs to intercept outgoing calls.
• Routes calls through a cloud telephony provider (Exotel).
• Replaces real phone numbers with virtual caller IDs, masking both caller and receiver MSISDNs.
• Automates proxy number assignment so users do not manually dial virtual numbers.
• Uses a backend with secure authentication (JWT, encryption, HMAC, nonce-based protection).

Key technical contributions

• Integration of Android telecom APIs with a cloud proxy call system for end-to-end number masking.
• A safe asynchronous call-handling design to avoid Android app crashes (ANR issues).
• Secure communication protocols for call initiation and webhook synchronization.
• A detailed measurement framework for analyzing call setup latency.
• A privacy evaluation method using SIP packet analysis to confirm real number hiding.

Threat model and security

The system assumes strong adversaries (network observers, malicious apps, partial backend compromise) and defends against:

• Number harvesting
• Session hijacking
• Metadata correlation
• API replay attacks
• Webhook spoofing
• Insider database threats

It relies on:

• TLS 1.3, AES-256 encryption
• Short-lived JWT tokens
• HMAC-SHA256 verification
• Virtual number pools
• Encrypted call mapping storage

Research gap addressed

Compared to existing systems:

• VoIP apps protect content but still expose metadata or depend on app-to-app calling.
• Cloud masking services require manual number handling.
• Android dialers do not support transparent privacy routing.

PP-SCMS claims to be the first system that combines:

• Native Android dialer integration
• Automated cloud-based number masking
• Full PSTN compatibility
• Strong backend authentication and encryption
• Transparent user experience

Conclusion

This paper has presented PP-SCMS, a privacy-preserving smart call management system that addresses the structural MSISDN disclosure problem in Android telephony by intercepting calls at the ConnectionService boundary — the earliest enforceable extension point in the Android telephony stack — and routing them through a cloud telephony privacy layer. The system is built on a formally specified threat model with eight identified attack vectors and corresponding countermeasures, two novel protocols (SCIP and WSSP) with explicit replay-prevention and webhook authentication mechanisms, and a database design that enforces cryptographic separation of call metadata from PII. Empirical evaluation over 150 test calls across three Indian carrier networks demonstrates a mean call setup time of 1,840 ms (660 ms overhead over the native dialer, attributable to the cloud routing pipeline), a call-drop rate of 1.8%, and zero real-number leakage verified at the SIP packet level. The 79.3 SUS score confirms production-grade usability. A latency decomposition model identifies T_pstn as the dominant contributor (67%) and guides future optimization priorities. The primary scientific contribution is the demonstration that Android\'s native telephony APIs — previously studied primarily from a security-audit perspective — can be repurposed as a privacy-enhancement mechanism through careful integration with external cloud telephony infrastructure and a well-designed authentication and logging backend. We expect this architecture to serve as a reference design for enterprise and consumer privacy-preserving telephony applications.

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Copyright

Copyright © 2026 Pawan Kumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.