Missed calls during sleep mode

I have googled this problem, and have discovered lots of people out there who have this particular issue with the iPhone missing calls when in sleep mode. When the phone is awakened, a message will display showing a missed call, but the phone never rings when in sleep mode! This is very annoying. The only thing I have found as a work-around is to set the Auto Lock feature in settings to Never, and just keep the phone active all day, which of course runs the battery down. I have gone to the Apple store, and they checked the phone out and said they thought it might be a SIM card issue and suggested taking it to AT&T. AT&T checked it out, had no clue, and said to take it to the Apple store. I have reset the phone and tried the restore feature, both of which did not resolve the issue. The phone is 3 months out of warranty now, and Apple will do nothing about replacing it. It took me 3 months to figure out why I was missing calls I am so frustrated. Apple really needs to fix this issue and pay attention to the discussion boards out there. Anybody out there have any other suggestions to fox this?