You need to assign the CO line to ring to a phantom UNUSED analog station as a private line with no phone attached. The station must then be covered to voice mail. Set the coverage forwarding on the Station to 1 ring.
In the Intuity, go to Auto attendant routing field and set the extension number in the first field to the DN of the phantom station, in the next field right type login and press f3 to save.
The Legend system talks to the Intuity in Mode Code and this will give you the user login prompt.