Here is what you do. Takes a little programming but it should work.
Create an analog line and don't put any hunting or forwarding on the line, or increase the number of rings to something like 10.
Then get the bell/light and put it on the analog line.
Then on the voicemail box set remote notification to that analog line. Then when a voicemail message is left the system will call that number and the light will go off and someone will have to call the voicemail system to check the messages.
If you want to get fancy, set a pickup group with the analog line you create and the phones where the light/bell will be, then when the light goes off they can pickup any phone in that area and use call pickup to grab the remote notification and get the voicemail.
This way you just need to get a loud bell or light that works off an analog line. and those are cheap.