A problem with the handling of UDP network packets is leaving T‑Mobile US customers unable to make FaceTime or WhatsApp calls with the latest Apple iOS beta.
Various netizens say they are unable to make video calls on their T‑Mo handsets when running the latest iOS beta build. WhatsApp calls are also reportedly dropped. Other carriers have not been reported to have the problem.
T‑Mobile US has yet to comment on the matter or offer any explanation to The Reg, but the issue appears to involve the way the telco's network handles UDP over IPv6 with the iOS 9.3 beta 2 release.
The beta triggers a carrier update that runs all of an iThing's LTE mobile broadband connections over IPv6. It appears T-Mobile's NAT64 implementation and Apple's networking software aren't playing nicely together, causing UDP packets to be dropped. This is what's causing FaceTime and WhatsApp to fail.
While most applications use TCP, streaming apps including FaceTime run over UDP, where the occasional packet loss takes a back seat to faster exchange rates. Users have reported that by running their LTE traffic through a TCP-based VPN, the issue is remedied.
It goes without saying that anyone running beta software should expect to encounter these sorts of bugs, and as such regular backups are encouraged, particularly before installing the beta profile for the first time. T‑Mobile US customers who wish to avoid the problem can opt out of the updates or revert to the last general release build by restoring their phone.
Apple has long made its iOS beta builds available to developers. Last year they began allowing normal users to run the builds, with the caveat that the beta OS is usually far less stable than the regular releases. ®