Just under two weeks after launching iOS 12.3 and a few days after releasing the second beta of iOS 12.4, Apple today released the iOS 12.3.1 (build 16F203) for owners of iPhones, iPads, and iPods touch.
According to the release notes, the new iOS 12.3.1 release has been released to fix bugs related to VoLTE calls (which could prevent such calls from being made / received) and iMessage (one in which messages from unknown senders could appear). in your conversation list, even with the filter to block such messages enabled, and another that could link to notify a message from unknown as spam disappears).
To upgrade iOS, just go to General Settings Software Update and install the update.
Here are the deep links for iOS 12.3.1 download, for those who prefer to apply the update manually through iTunes: