iPhones missing calls while sleeping (locked)

Several users are reporting an issue where the iPhone inexplicably does not actively receive phone calls while in sleep/locked mode (invoked by pressing the sleep button on top of the unit, or the Auto-Lock function in Settings > General). When this problem occurs, users are sometimes notified that they have missed calls when the phone is unlocked (awoken), though the phone never rings, vibrates, turns on the screen or provides any other indication when the calls were actually incoming.

In a separate but related issue, some iPhones, in some cases will notify that a call is incoming and present the "Slide to Unlock" screen, but attempting to move the slider has no effect, with the interface being unresponsive.

Some users are apparently having their iPhones replaced by Apple because of this issue. Apple Discussions poster Leshi writes:

"Went to see a Genius at a local Apple Store today with the problem after trying everything possible (e.g. reset, restore) and multiple searches on this support discussion board. After a quick evaluation and restore, they swapped the iPhone for a new one! Now it's fully functional."

Similar issue? Please let us know.