Specific version of Ericsson vMME found culprit of today’s 11 nation cellular crashes worldwide including O2 UK, SoftBank JP, Mobifone VN

https://www.ft.com/content/778469aa-f934-11e8-af46-2022a0b02a6c https://www.softbank.jp/corp/set/data/group/sbm/news/press/2018/20181206_02/pdf/20181206_02.pdf

vMME or Virtual Mobility Management Entity is an Ericsson product to provide MME functionality inside EPC or evolved packet core, virtualized.

The problematic vMME software caused outages in operators across 11 countries, according to Ericsson, cited by SoftBank. This version was 9 months old, and was rolled further back to resolve the issue.

TL;DR in classic 3G terms, HLR in Core crashed and MS couldn’t authenticate or BS couldn’t connect so everyone went no signal.