T, welcome
I see you found the "post new topic" link.
There are a few ways to do this.
If your referring to after a call has been answered by an attendant then all you need is a (VTR) voice mail transfer key on the set.
While during a conversation you would press the VTR key, press a DSS key or dial an Ext. with voicemail and hang up.
Rather the list the other options, is this what your looking for?
MrG