Today Apple has released iOS 12.3.1 to all of the supported iPhones and iPads. As its name implies, this is in no way a big release, it's actually just meant to fix a bunch of bugs.
Those bugs have to do with VoLTE calls - specifically there was an issue that could prevent making or receiving such calls. The Messages app got two fixes too, one of a bug that could cause messages from unknown senders to appear in your conversation list even if you had the Filter Unknown Senders option enabled, and the other of an issue that could prevent the Report Junk link from appearing in Messages threads from unknown senders.
And that's it, the entirety of the changelog for iOS 12.3.1. While it's definitely not a big update, you should still install it as soon as possible, especially if you were affected by one of these bugs or could be - like if you use VoLTE, for example, or receive a lot of message spam from unknown senders.