  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.
