missed call alerts

I'm not looking to debate with anyone, just need a solution. I need my PHONE (hence the phone part in iPhone), to alert my to missed calls with an audible alert. Yes I know it shows you missed calls on the screen, but if you have your phone auto-lock after five minutes you never see those messages on the screen unless you hit the home button and look. I need an audible alert so when the screen is off I can still know I missed a call. After all this is a phone and phone is its most important function, that's what we all pay at&t for.
I do not have a land line, haven't had one for 6 years, I only use a cell phone.
Reason I need this is last night a family member was rushed to the hospital. I was in the other room and missed the 3 attempts to call me. A voice mail was left, but I didn't wake the phone up until 2 hours later and finally saw the missed calls. I rushed out to the hospital immediately once I listened to the voice mail.
So my questions are:
1) Is there an app, either free or pay, that I can get that will give me audible alerts to missed calls? (at say a specified time interval)
2) If no legal app is available, is there a app available if I hack the phone?
3) If 1 and 2 are not an option, what is the policy of at&t to switch from this "phone" to another phone? (I've been with att for 8 years, don't want to drop them, just need a different phone if I can't get audible alerts)

Thanks for any help you can provide.