I have hopefully a quick question; what dictates how many rings a subscriber hears before Unity answers? Originally Unity answered immediately, now subscribers here 2 rings before Unity answers and prompts for pin.
You can check the status of the voicemail ports from the Unity server. Right-click the Unity icon on the server's systray and select Launch Status Monitor. When this opens, click on the second icon, which is Port Status (looks like a phone with lines). Check to see if you have any ports in the State Column which show Busy with a high Time Count. Occasionally you may see one or more ports with 1000+ in the Time Count, indication of possibly a hung port. You can try to Reset the hung port, but I've found it more successful to stop/restart Unity, or if that doesn't work, to reboot the server.