In Microsoft Office Communicator the offline status is not a default state. It is configured by the administrator and even if it indicates that you can't contact the person that use it, you need to know that the user is actually logged on, and despite appearances, is able to take part in conversations and conferences. From what it seems, the users that are using this state are busy and they don't want to be disturbed.
If you want, you can enable this state by creating a new registry key and registry value on the computer running Communicator Web Access. Log on to the PC using Administrator rights, go to Start, type regedit and open the utility. Navigate to HKEY-LOCAL-MACHINE > Software > Policies > Microsoft, right-click Communicator and select Key from the New list. Once the new key has been created, rename it yo Policy. Also, you need to know that this key may already exist. In this case, ignore the above steps, right-click the Policy key and from the New list select DWORD (32-bit) Value. Rename the new value to EnableAppearOffline, double-click its icon, type 1 in the Value data box and click OK.
Note: If an "offline" user wants to communicate with you, but you can't contact that person, simply remove the software from your computer (Control Panel > Uninstall a program) and install it again.