You could do a 2rd party call forward to the auto attendant.
From your phone, dial **69<ext><VM extension> (For the rest of you techs that are say'n "**69 what the heck is that?" My company set up this system so I know the codes.
)
<ext> is the hunt group the calls ring into - probably a series of keys on the phone.
<VM extension is the voice mail hunt group>
This will either drop you into the auto attendant or the voice mail box of the <ext>. Depending on which one, rerecord it's greeting.
I don't know your exact set up so I guessing a bit.
Have you tried just putting the system in to night mode?
Ralph