Apple has issued an unexpected iOS update. Version 12.3.1 was just released, with no public or developer beta testing preceding it. It’s unusual for Apple to release an iOS update on a Friday except in cases of critical, high-profile bugs.
Fixing bugs is exactly what this release does. The official release notes for this update are brief:
Fixes an issue that could prevent making or receiving VoLTE calls.
Fixes issues in Messages that could cause messages from unknown senders to appear in your conversation list even though Filter Unknown Senders is enabled.
Fixes an issue that could prevent the Report Junk link from appearing in Messages threads from unknown senders.