https://wccftech.com/download-ios-12-3-1-now-available-with-volte-and-imessage-fixes/

Apple has released iOS 12.3.1 for iPhone and iPad. This is a bug fix release that patches issues pertaining to VoLTE and iMessage.

We are basking in the glory of iOS 12.3 at the time of writing, and already, Apple has seeded the next software update, iOS 12.3.1. This is not an earth-shattering release by any means as it fixes a few bugs related to VoLTE and iMessage. If you have been having issue with VoLTE on certain carriers, this issue should fix that problem. Furthermore, you might have noticed that the iOS’ junk message filter does not work as intended at times, allowing junk messages to trickle through to your main inbox, and the latest update fixes that issue.