There are about 3 things that control whether you can do this or not. I guess when it was moved, you were given a different extension or the system was reprogrammed and the feature was eliminated.

Supervisor barge-in or executive busy override can be silent or give a warning tone. If you have access to programming it's in FLASH 50 page A button 12 as outlined in the manual on page 224 forward. Read the procedure carefully, as you also have to remove the override tone if you want silent observing.

It's an installer feature, so if you've never programmed the system or don't have installer access, then you ought to call whoever moved the system back and have them put the feature back in.

Here's a manual http://www.phonearena.com/htmls/Motorola-Rival-phone-review-pr_3561.html

Carl