A Google Voice outage was caused by expired certificates used to encrypt all Google Voice traffic. Google engineers are taking several measures designed to prevent future outages. In December 2020, Google suffered a global authentication system outage that affected most of its consumer-facing services, including Gmail, YouTube, Google Drive, Google Maps, and Google Calendar. Google is also taking steps to prevent a similar issue from occurring again and decrease the impact of future incidents, the engineers said in an incident report published on Friday.
Source: https://www.bleepingcomputer.com/news/google/recent-google-voice-outage-caused-by-expired-certificates/