You can do that now. Under "Call Handling & Forwarding" for an extension select the "Incoming Call Information" menu choice. In the bottom half of the screen you'll see "Play announcement before connecting". Set these so that you will receive information about the call when you answer it.
Your caller will still be on hold at that point in time. It will give you the option of pressing "1" to accept the call or press STAR (*) for more options. The option of "2" will send the caller to voicemail. You will be advised that you can interrupt the voicemail and take the call by pressing "1" during the voicemail. Meanwhile you will hear the caller leaving the voicemail, and should you elect to take the call, you can press "1" to break off the voicemail and go live with the call. Otherwise you can hang up at any time. As long as you don't press "1" during the voicemail, you will not be connected to the caller.