You DO have to make a change.
In WebPro, go to section 10-03 and make sure the port is set to "NOT SET".

Plug the SLT adapter into the digital port.

Refresh 10-03 in WebPro, and you should see that the port has been changed from "Not Set" to "SLT" (or "SLT Adapter", I forget what actually shows).

At that point, you'll be able to plug your analog conference phone.

If you do not inspect 10-03, you may get a clicking/popping sound if a TDM set was plugged into that port previously, etc.