I just got around to connect my DS01 and RGR04 module to my Ultra II test system. I've got a phone connected to TEL_OUT and when I press the DS button the phone starts ringing as per normal. When I lift the handset then I can still hear the ringer trying to 'phone' and there is no comms between the phone and the DS.
What I think is happening is that Comfort doesn't detect that I lifted the handset. I even tried a different phone but with the same result.