Phone doesn't ring or show missed call, later a voicemail appears
I had the same issue. You will have to change your phone notification settings. Go to "settings," then "call", then "phone settings" under "other call settings."  Scroll all the way to the bottom of the call setting screen and select "phone notification preview" (To show caller ID for missed calls and voicemails on locked screen and status bar).

Most likely, you downloaded an app on your phone that adjusted your phone settings. I hope this helps.