I've had this problem several times in the last few weeks. I'll get a voicemail notification even though I've had my phone on my desk, well within hearing range.
In short, I get a call, the phone doesn't ring, the call goes straight to voicemail, and the call log does not show a missed and/or incoming call. This has happened with several different contacts.
The ringer is on High (I use Volume Locker so no problem with accidentally turning it down or muting), the conatcts do not have the "straight to voicemail" option checked, and most of the time the phone works fine. It's very random.