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.