Apple today released iOS 12.3.1 which:
• 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 is available to download right now via Settings > General > Software Update.