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.