Teams users around the world have reported weird “FCM Test messages” on their mobile devices. JAMK users have also received these messages.
Earlier this week, Google Hangouts users reported this same issue. It was and still is a vulnerability in Google’s Firebase Cloud Messaging service that apparently can now be exploited against Microsoft Teams users as well. For the sake of certainty, thes enotifications should probably be left unopened. Microsoft is investigating the issue and probably releases official announcement and fix in the near future.