The iOS 12.3.1 release does not appear to fix any security flaws, but rather deals with issues relate to VoLTE calls, and two separate issues in the Messages app.
As detailed by Apple on its iOS updates page, the iOS 12.3.1 release 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, and finally, fixes an issue that could prevent the Report Junk link from appearing in Messages threads from unknown senders.
To download and install iOS 12.3.1 on your iPhone or iPad, head to Settings > General > Software Update on your device, download the update, and then install it. It is recommended for the iOS device to have more than 50 percent battery before the update process is started. As we mentioned, no security fixes are listed by the company.
iOS 12.3.1 can be expected to be the last bug fixing update ahead of iOS 13, which will be unveiled by the Cupertino-based company at its Worldwide Developers Conference (WWDC) 2019 keynote on June 3.
Several new features are expected include a Dark Mode, UI tweaks, new animations, and swipe typing support on the official keyboard. As we mentioned, Apple released iOS 12.3 earlier this month, bringing the redesigned Apple TV it had announced back in March alongside its Apple TV+ service.