Step 3: Click on the Extension Manager menu item.
Step 4: Click on the Extension sub-menu item.
Step 5: Select the correct extension.
Step 6: Click on the Options tab at the top.
Step 7: Ensure Call forwarding is disabled.
Step 8: Ensure Call waiting is disabled.
Step 9: Ensure a busy message is recorded via the *85 feature code.
You need to be on a call for this feature to work (It does not work for no answer, only busy)
Once all these conditions have been met, dial the extension. If they are on a call can dial "0".