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

In current cellular network generations (4G, 5G) the IMS (IP Multi-
media 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 do-
mestic 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.