We're still new to the 6900 phones and are working through some of these issues ourselves.
Just for anybody who might still be wondering, or who might be new to the Office 250 and the 6900 phones, the instructions Tech Electronics gave do work.
We set up a Voicemail DSS key that points to 2500 so that when you have a caller you can press Xfer, the VM DSS key, dial the mailbox No. and immediately press goodbye button or hang up.
If you press the goodbye key immediately, the caller will hear most, if not all of the users greeting. Still, that is dependant on how quickly the system is able to route the call.
Hope that helps any newbies to the system.