I've had this issue from time to time on every WM device I've used. It happened last week on my stock Sprint Samsung Intrepid, at home, with great signal strength and no recent PRL update, etc. Just got a voicemail popup and the phone didn't ring, didn't vibrate, nothing. So there could be a number of causes; it may not relate to the TP2 specifically. Personally I think it's a WM thing, particularly since I've seen it on about 5 different WM phones on VZW and Sprint, but I don't recall it happening with the non-smartphones I used to use.
Why is an Intrepid user lurking in here...TP2 is on its way!!!!