Check the event log in Unity to see if the Partner server is offline or can not connect. If the partner mail server (exchange or domino) is offline, the voicesmail will get stored on the Unity server until delivered.
The other question I would ask is whether or not you mean that someone can check voicemail, it asks for their password, there is a delay and then Unity finally says "you have x number of messages." If that is the delay you speak of, it's usually a problem with network connectivity between Unity and Exchange if Exchange is on it's own server and not hosted on Unity. Here is a great doc regarding delays in conversation for Unity and Exchange. Please go through it.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.