Phone doesn't ring or show missed call, later a voicemail appears

I have an AT&T Samsung Galaxy 2 and have had a sporadic problem: A person calls me, my phone doesn't ring, kicks them over to voice mail, doesn't show as a missed call, and then the voice mail shows up on my phone as much as an hour later. I have tested this from my land line and sometimes the phone does everything it's supposed to and other times doesn't ring and doesn't log a missed call but records a voice mail.