Apparently the iPhone seems to have troubles with ringing in sleep mode for some users. But read for yourself:

My phone doesn’t ring once it goes into sleep mode. No telling how many business calls I missed. I had the phone less then 24 hours and ATT wouldn’t replace it. Took it to the Apple store, now I have to wait until they get more phones in for a replacement. I can’t believe this uttler lack of service from both ATT and Apple. Never again.

This report has been submitted through the iPhone complaint form.