You would have to use something in Outlook to do this. You can create an email message and before you hit send, go into options and select when you want it be sent. I do this all the time with Network Planned outages. I just create the emails, set my options and then they are sent out in the night or AM.
Essentially with Outlook and UM configured, you can create a voicemail, save it, then set your options to send at a later date or time. I have tried it with a voicemail, but if it works with email, you can use this feature with vm.
If your infrastructure is VMO, it might be a little tougher to do.
Unity has the ability to record and send a message as a "future delivery" on a specified time and date. This future delivery message will not ring the phone, but will light the message lamp etc. Here is the info;
When you send a message, Cisco Unity sends the message to your intended recipients without ringing their extensions. You can send voice messages to Cisco Unity subscribers, and to public and private distribution lists.
You can address a message to more than one recipient or type of recipient. You address a message by using the phone keypad to either spell the subscriber name or to enter the extension of the subscriber. When you address messages to private lists, you must use number entry.
When you send a message by phone, you can choose from the following special delivery options:
Urgent Mark a message urgent so that Cisco Unity sends it before regular messages.
Return Receipt Request a return receipt so that Cisco Unity notifies you when the recipient opens the message.
(Note that you may not be able to receive all types of receipts, even though the Cisco Unity phone menus allow you to request them. Ask your Cisco Unity administrator whether Cisco Unity is set up to send receipts to subscribers.)
Private Mark a message private so that the recipient knows to treat it confidentially.
***Future Delivery*** Mark a message for future delivery so that Cisco Unity waits to send the message on the day and time that you specify.***
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...