Apple launched iOS 12.3.1 as:
. • Solves an issue that can prevent you from receiving or receiving VoLTE calls
. • Addresses issues in Messages that can cause messages from unknown senders to appear in your call list. even though Filter Unknown Senders is enabled
. • Releases an issue that can prevent the error message from appearing in Messaging threads from unknown senders
iOS 12.3.1 is available for download now via Settings> General> Software Update.