I just uninstalled the official Twitter app. I sure hope it works. I was considering returning my Charge because of this issue. I'm missing call all the time because of this. And it does seem to be isolated to Samsung phones, my wife's Incredible 2 doesn't have this problem. Plus I've never encountered anything like this on my Nexus One.
Of note: The official twitter app does install a notification sound onto the phone. Maybe that file, or the location or method in which it installs (or something related) is at the root of the issue. If uninstalling the Twitter app doesn't help, I'm going to look into other apps that install sounds to the device.
Don't know, but if I can't resolve this issue before my return window closes, I'mma have to exchange this phone. Samsung just doesn't have a good enough reputation for standing behind their products to be left on the hook with a phone that doesn't ring reliably.
But I'm glad all you helpful folks are here to diagnose these things.