When you dial the conference bridge number (89000) from the softphone a pop-up box appears on the bottom-right of the application which says 'CTI Assisted'
Clicking on the green phone will initiate the conference however there will be no dialpad pop-up when you click on the dial pad icon, so a PIN cannot be entered
There are two workarounds:
1) Enter the pin number on your keyboard immediately on entering the conference. Don't forget to type the hash after the pin. If you type anything else, this method will not work, but you can drop the call and try again.
OR - if this conference was set up by you:
2) Click on the triangular conference button and click the dial button to the right.
Clicking on the green phone icon to start the call (see above) will still bring up a box but this time tagged 'Conference', click on the green phone.
Now when the dial pad icon is clicked on, the dial pad will be visible: