That's news to me. Only way I could see that working is to have a skillset that overflows to an analog extension when nobody is logged in. I can't imagine trying to have a user use a flash button to enter F904, then their ID, password, etc.
From the 3.6 docs, which looks just like what hawks found in the 450 docs:
Non display telephones, such as telephones attached to an analog terminal adapter, cannot be used for Call
Center agent tasks. We recommend that you read the telephone user card for your telephone before proceeding.