There is no such thing as "automatic hold", but there should be a workaround.
The first thing that comes to mind: put an analog station on a Secondary Call Key set to zero rings. Make the analog station a member of the hunt group, and have the hunt group recall to the analog station after 18 seconds. No forward path or attendant on the analog station.
The end user experience should be phone rings 3 times with the light flashing, then just flashing light with no ring. With no forward path or attendant, calls won't recall from the analog station. I don't think this will work with a phantom, but it's worth a try.