Introduction

With the increasing prevalence of Voice over Internet Protocol (VoIP) in both business and residential settings, it's common to encounter questions about differentiating between VoIP and GSM calls. This article delves into the technical aspects of distinguishing these calls, focusing on the capabilities of Asterisk, and offers practical advice for filtering and managing incoming calls.

Distinguishing VoIP and GSM Calls

Incorporated within this topic are financial institutions, technology majors like Google, Microsoft, and Azure that leverage similar technologies. These companies use APIs and databases to identify VoIP calls, ensuring their services are not abused by entities attempting to use VoIP numbers for verification or spam purposes.

Technical Underpinnings

Technically, once a call is routed through the main telephone interchange, there is no metadata to indicate the initial technology used to initiate the call. VoIP and GSM calls are converging, with the vast majority of legitimate phone traffic now VoIP-based. Major and medium businesses, as well as home landlines from providers like ATT, Spectrum, and others, are VoIP-centric.

The Problem with Blocklists

To illustrate, initially, some users tried blocking VoIP calls on their landlines, hoping to filter out spoofed spam calls. While understandable, this approach would make the landline largely unusable. Blocking legitimate VoIP traffic would render the phone device nearly useless as a communication tool. Therefore, it's crucial to differentiate between these calls in a manner that preserves communication integrity.

Using Asterisk for Call Filtering

Asterisk is a widely used open-source PBX (Private Branch Exchange) software, enabling sophisticated call management and filtering. Here’s how you can filter or block VoIP calls and keep only GSM calls:

Setting up Asterisk for Filtering

Define Call Routing Rules: Configure Asterisk to route VoIP and GSM calls through different trunk groups or extensions. This can be done using and files.

API and Database Integration: Use APIs with databases to identify VoIP numbers and block them. This can be achieved through custom scripts or integrations with third-party services.

qScan Utility: Utilize the qScan utility provided by Asterisk to scan and validate incoming calls. While not automatically blocking, it can provide insights into call types and origin.

Third-Party Services: Consider integrating with third-party call screening services that can help identify and filter VoIP calls.

Safeguarding Your Asterisk System

Given the potential for security threats, it's essential to ensure your Asterisk system is secure and well-maintained:

Disable Guests: Eliminate SIP guests and ensure that only authenticated users can access your system.

Firewall Configuration: Harden your firewall with current IP Tables and ensure it is up to date.

Disable Anonymous Call Features: Disable any anonymous call features in both the system and individual extensions.

Disable Direct SIP Calling: Unless explicitly required, disable direct SIP calling to prevent unauthorized access.

Conclusion

While the differentiation between VoIP and GSM calls is technically challenging, employing Asterisk’s advanced features and supportive security measures can help you filter or block VoIP calls effectively. This approach ensures that your communication infrastructure remains robust and secure. By integrating custom scripts, leveraging APIs, and implementing stringent security practices, you can achieve a balance between managing legitimate and spam calls.

Further Reading

Asterisk Official Website

VoIP vs GSM

ATT Considerations for VoIP over GSM