Apple has released iOS 12.3.1, this update includes bug fixes for voice over LTE calls as well as iMessage filtering and reporting, as notes by 9to5Mac:
- 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
iOS 12.3.1 comes almost two weeks after the official release of 12.3.