You'll first need to determine if message recordings are being cut-off during the recording or if callers are able to leave full messages but the messages then get trimmed or clipped. If callers are cut short while recording, then you'll next need to determine if they are being hung up on (Unity falsely detected disconnect) or if they are being prompted with the after message menu (Unity falsely detected silence.
The follow tech tip might help you out. It includes discussion on topics such as, "Callers are reporting being disconnected (hung up on) while they are recording a voice message."
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_tech_note09186a0080093c1e.shtml
