Samsung Moment : Voicemail Greeting / Off & On

A friend recently got a new Samsung Moment. Since then, he reports that callers sometimes get his personal voicemail greeting, but sometimes just get a vanilla greeting with the phone number, or perhaps an inability to leave a message at all. We're trying to figure out why.

1) Could it be related to the fact that the Samsung Moment has Visual Voicemail? My friend doesn't use the Visual Voicemail program on the phone (yet), but rather dials 1, just as he used to, to both set his greeting and hear his messages. My understanding, however, is that he was switched to a Visual Voicemail-capable server anyway.

2) Could it be related to roaming? I wonder whether his new job is in an area with poor Sprint service, where his phone is often going into roaming. This would also explain why his battery is not lasting the full (overtime) workday.

3) Could the separate server used for Visual Voicemail be downright flaky, constantly flopping up and down, or perhaps over-busy? Wouldn't other customers of that server complain if so?