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

TL;DR

The paper investigates the global deployment of VoWiFi and IP-based geoblocking by mobile operators, revealing that many block VoWiFi access from foreign locations at the DNS or ePDG/IKE level with potential impacts on emergency calling. It employs a global measurement framework using VPN/cloud-based vantage points to map ePDG DNS records and to probe IP-based access via IKE across 219 countries, yielding thousands of domain-IP mappings and hundreds of ePDG endpoints. The study finds substantial geoblocking: approximately 12.5% of IPv4 domains and 65.2% of IPv6 domains exhibit large-scale roaming blocks, with regional concentration in Europe and Asia, and notable country-specific rules; it also documents DNS-based blocking instances and asymmetric domestic-roaming behaviors. These findings highlight policy, security, and public-safety implications for VoWiFi and roaming, and the authors contribute an open measurement toolkit to facilitate future, reproducible investigations.

Abstract

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.

Figures (6)

• Figure 1: An Indian operator states in the FAQs Airtel2023Blocking that VoWiFi cannot be used during international roaming.
• Figure 2: (Simplified) LTE network architecture for VoLTE and VoWiFi.
• Figure 3: To connect via VoWiFi, the client fetches the ePDGs IP address via the DNS server and starts the IKE negotiation with the ePDG.
• Figure 4: A containerized architecture isolates independent measurements and enables parallel execution for simple upscaling.
• Figure 5: Leveraging VPN and cloud services, we reach de facto worldwide measurement coverage.