A new mobile network security investigation has raised concerns about the amount of technical information that can potentially be revealed when someone receives a phone call. Researchers found that, under certain network configurations, an incoming call could expose a smartphone’s 15-digit IMEI number, device model and operating system information before the recipient even answers.

The findings are particularly significant because the information can be exchanged during the technical process used to establish a Voice over LTE (VoLTE) call. While the issue does not mean that attackers can automatically take control of a smartphone, exposed device information could help them identify potential targets and determine whether a device may be running vulnerable software.

The investigation was conducted in Germany and involved more than 70 test calls across major mobile networks.

Incoming Calls May Reveal More Than Expected

When a smartphone receives a call over a modern LTE network, several technical exchanges take place before the conversation begins. These exchanges allow the networks and devices involved to establish the connection.

According to the investigation, some of these communications can contain information about the receiving smartphone that was not intended to be exposed to the caller.

Researchers found that the IMEI, a unique identifier associated with a mobile device, was exposed in some cross-network calls involving German networks operated by Telekom and Telefónica’s O2. Smartphone model and operating system information was also observed in certain tests involving Telekom and Vodafone.

The tests demonstrate that the recipient does not necessarily have to answer the phone for the information to potentially become available.

What Is an IMEI and Why Does It Matter?

The International Mobile Equipment Identity (IMEI) is a unique identifier used to identify a cellular device. The GSMA describes the IMEI as a 15-digit identifier associated with a device operating on a mobile network.

The IMEI is normally used by mobile networks for purposes such as identifying devices and helping operators manage stolen or blocked handsets. The first portion of the number, known as the Type Allocation Code (TAC), can identify the manufacturer and model family of a device.

That makes an exposed IMEI potentially more useful to an attacker than an ordinary piece of technical information. When combined with a device model and software version, it can provide a clearer picture of the smartphone being targeted.

Users can learn more about how device identifiers work through the GSMA’s official IMEI information.

The Problem Is Linked to VoLTE Call Setup

The reported exposure is connected to Voice over LTE, or VoLTE, a technology that allows voice calls to be carried over LTE networks using IP-based communications.

VoLTE has become an important part of modern mobile networks because it allows operators to provide voice services over 4G infrastructure. The GSMA describes VoLTE as an industry-standard approach to delivering packet-based voice services through LTE networks.

However, the investigation highlights how differences in network configurations and the information exchanged between networks can create unexpected privacy and security risks.

In certain cross-network scenarios, technical information associated with the receiving device was reportedly passed through the call-signaling process and became visible on the caller’s side.

More Than the IMEI Could Be Exposed

The security concern is not limited to the IMEI.

Researchers also identified cases in which the smartphone model and operating system version could be determined. That information can be valuable to someone attempting to profile a target.

For example, knowing the exact smartphone model can help an attacker determine which hardware and software environment a victim is using. An operating system version can provide additional context about whether the device may have missed security updates or could be affected by known vulnerabilities.

Cybersecurity experts therefore consider the exposure a potential reconnaissance risk. The leaked information itself does not automatically compromise the phone, but it could make subsequent targeted attacks more informed.

GSMA Warns Mobile Operators

The findings prompted the GSMA, the global industry association representing mobile operators and the broader mobile ecosystem, to alert more than 1,000 operators to review their network configurations.

The organization reportedly advised operators and technology vendors to examine the information being transmitted during call signaling and remove unnecessary device-related information where appropriate.

The GSMA has extensive technical documentation covering VoLTE implementation, interoperability and network testing. Its guidance emphasizes the importance of consistent implementation across mobile networks and devices.

This is particularly important as operators continue moving voice services away from older 2G and 3G technologies toward IP-based networks.

German Networks Respond to the Findings

The investigation focused on Germany’s major mobile networks and found different types of information exposure depending on the network and call scenario.

According to reporting on the investigation, German operators indicated that measures had been taken to address or reduce the issue. Vodafone said its tests did not show the same IMEI exposure, while other operators disputed or explained aspects of how the information was transmitted.

The different responses underline an important point: the problem does not necessarily affect every mobile network or every smartphone in the same way.

What Does This Mean for Smartphone Users?

For ordinary smartphone users, the discovery is primarily a network-level security concern, rather than a reason to assume that every incoming call is exposing personal information.

The reported issue depends on specific network configurations and call scenarios. There is also no indication that simply knowing an IMEI gives an attacker direct access to a smartphone.

Nevertheless, users should continue following basic mobile security practices. Keeping the operating system and apps updated can reduce the risk of attacks that rely on known vulnerabilities. Users should also be cautious about suspicious calls, messages and attempts to obtain personal information.

Mobile operators, meanwhile, have a more direct responsibility to ensure that unnecessary technical information is not exposed during network signaling.

A Warning for the Future of Connected Networks

The investigation highlights a broader challenge facing the telecommunications industry. As mobile networks become increasingly software-driven and interconnected, security problems can emerge not only from smartphones themselves but also from the way different networks communicate.

VoLTE provides faster call establishment and enables modern voice services, but its security depends on correct implementation across devices, operators and interconnecting networks.

The reported IMEI exposure serves as a reminder that even something as routine as receiving a phone call can involve complex exchanges of technical information behind the scenes.

For users, the immediate lesson is simple: an incoming call may involve far more network data than the conversation itself reveals. For mobile operators, the findings reinforce the need to minimize unnecessary device information in call signaling and regularly test network configurations for privacy and security weaknesses.

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