Since I am now using the Aastra Response Point system I figured I would connect my Jabra T5330 cordless headset - that I hadn't been using since switching to the Aastra system. This headset base can connect to the phone two different ways - depending on what the phone supports. The most universal (are there different degress of universal? I don't think so) way is putting the headset in series in between the telephone base and the telephone handset. This is the way I used the headset with Syspine phone.
The Aastra phone has a headset jack on the underside of the phone. This makes it possible to just connect the headset to the telephone base unit independent of the telephone handset cord. When I tried the headset connected this way I was not able to get any dialtone. After hunting around I finally noticed that the Speaker button on the telephone base really said Speaker/Headset! Gee, this looks relevant. Anyway, I am on a conference call now, using the headset, and do not have to have the telephone handset 'offhook' to be able to have the call. The red light by the Speaker/Headset button that is on solid when I use the speakerphone is blinking red to indicate I am on the headset. FWIW.