I think the use of VM Pro is the best way to go.
Create a user called NightIT, enable "Ex directory", add a usershortcode : SC=*90 TN = "ITModule" Action = Voicemailaccess LineID = 0, divert this user to *90 and check the "Divert huntgroup" option.
Create a huntgroup "OvNightIT", type hunt and add the user "NightIT" as a member.
Create a group like "IT", put in the members, disable voicemail and add OvNightIT as a destination in the nightservice mode.
In VM Pro create a user variable called ITOnDuty.
Create a module named "ITModule", add a menu with only one DTMF option and a timeout of three seconds.
TimeOut:
Connect the TimeOut to a Test User variable action, in the properties page select the ITOnDuty variable and set 0 as a value. Connect True to a divert action and put in the external number of the first person.
Connect the False end to a new Test User variable action, in the properties page select the ITOnDuty variable and set 1 as a value. Connect True to a divert action and put in the external number of the second person.
Connect the False end to a divert action and put in the external number of the third person.
DTMF
Connect the DTMF end of the menu to a new menu and add $ as Touch Tone.
Connect the $ end to a Set Uservariable action. In the properties select the variable ITOnDuty and give ot a value of $KEY
How does it work?
If the group "IT" is set in the night service then calls are routed to the VM "ITModule". Caller enters the first menu were you can add a wav file like "Please Hold, you will be forwarded" or whatever, the caller is not aware of any option to choose so it will leave the menu with a timeout and the call will pass the variable checks to a divert action.
By default all user variables are set to zero so if the VM is rebooted the the first caller will be called.
If you want to change it just call the group IT in nigtservice and during the wav file press the DTMF key you programmed and then select wich user must on service : 0, 1 or 2. Because of this menu it can be changed from a remote location during night hours, always usefull if the one person on duty can't answer calls for any reason ( ill, drunk, stoned, sacked or whatever ).