Apple has released iOS 12.3.1 today with fixes for Messages and VoLTE calls.
According to Apple's release notes:
iOS 12.3.1 includes bug fixes for your iPhone and iPad. This update:
- 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
The update is available now on all supported devices.