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ACM MobiSys 2024 – Paper Accepted

Gabriel Gegenhuber’s new paper “Why E.T. Can’t Phone Home: A Global View on IP-based Geoblocking at VoWiFi” was recently accepted to the MobiSys conference happening in Tokyo in June 2024.

The paper exposes existing blocking practices within worldwide mobile network operators that prevent customers from using Wi-Fi calling during roaming.

The technical findings of the study reveal possible conflicts with prevalent net neutrality guidelines and consumer protection. Moreover, employed blocking rules could negatively affect emergency calling availability, which requires urgent action from responsible stakeholders. Therefore, Gabriel is currently disclosing the findings to regulators, emergency number associations and industry bodies.

The scanywhere measurement framework that was built throughout the study was made open source on github. It allowed him to automatically distribute his measurements to different vantage points in over 200 countries by leveraging cloud infrastructure and consumer-grade VPN subscriptions.

Title of the paper

Why E.T. Can’t Phone Home: A Global View on IP-based Geoblocking at VoWiFi


Gabriel Gegenhuber, Philipp É. Frenzel, Edgar Weippl


In current cellular network generations (4G, 5G) the IMS (IP Multimedia Subsystem) plays an integral role in terminating voice calls and short messages. Many operators use VoWiFi (Voice over Wi-Fi, also Wi-Fi calling) as an alternative network access technology to complement their cellular coverage in areas where no radio signal is available (e.g., rural territories or shielded buildings). In a mobile world where customers regularly traverse national borders, this can be used to avoid expensive international roaming fees while journeying overseas, since VoWiFi calls are usually invoiced at domestic rates. To not lose this revenue stream, some operators block access to the IMS for customers staying abroad. This work evaluates the current deployment status of VoWiFi among worldwide operators and analyzes existing geoblocking measures on the IP layer by measuring connectivity from over 200 countries. We show that a substantial share (IPv4: 14.6%, IPv6: 65.2%) of operators implement geoblocking at the DNS- or VoWiFi protocol level, and highlight severe drawbacks in terms of emergency calling service availability.

About the conference

The 22nd ACM International Conference on Mobile Systems, Applications, and Services seeks to present innovative and significant research on the design, implementation, usage, and evaluation of mobile computing and wireless systems, applications, and services.