have put a few 3rd party Vmails behind Legends.
Avaya does a lot of testing ports, "false rings" so you will need to make sure these tests are setup to do a disconnect. Otherwise it will do whatever you have setup for nothing pressed setting, usually go to operator. So they will get "no one there" a lot.
Doing a trace will help, but here are a few:
#06###
#07###
#08###
#10###
#11###
#00#(extension)## = login to mailbox