I re-installed a Merlin Legend and Messaging R.3 for a new customer. The system was used and I offered no guarantees, but the only goofy thing happening (or not happening) is that one user's MW light is not illuminating when she gets a message. I cannot find anything in the manuals to test it. (If you haven't guessed, the legend is not my specialty
the voice mail ports need to set to rotary enabled.
If it's only one user that doesn't get MW lites, check that you can turn it on and off manually, to be sure that the LED is good. Check the mailbox's "phone status", make sure that it says that "a phone is associated with mailbox XXX". If it is "no phone", it won't bother to try lighting the MW lite.
If you can't turn it off and on manually try another set, if that doesn't do it try another extenstion port.