I’m assuming you’re wanting to do this for a particular user, rather than system wide ?
You could set up a ring group specific for that extension, and calls to that extension would fail over to the ring group.
After x amount of seconds, calls in the ring group would fail over the users voicemail box.
Not very elegant, and it means setting up specific ring groups for this functionality, but it’ll work.
Chances are, quite a few callers will hang up when they hear the call transfer to the ring group, because they’ll think it’s going to voicemail, so be prepared to have your staff report ‘phantom’ or short calls to you.