The iOS 12.3.1 update is available on all eligible devices over-the-air in the Settings app. To access the update, go to Settings > General > Software Update. As with all iOS updates, iOS 12.3.1 is free to download, but it may take some time to propagate to all iOS users.
According to Apple’s release notes, iOS 12.3.1 addresses a handful of bugs related to the Messages app and VoLTE calls.
iOS 12.3.1 includes bug fixes for your iPhone and iPad. This update:
– 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
Apple did not offer a developer or public beta testing period for iOS 12.3.1, as is common for bug fix releases that are intended to address only a very few targeted areas and thus don’t require significant feedback from users before public release.