A telephony-over-LAN (ToL) system having a graphical user interface (GUI) wherein an authorized or guest user may be locked within a ToL window 3008, having full access to the ToL features, but denied access to other parts of the computer system. In such a system, the terminal user or subscriber may click on a "Guest" button 3013 on the ToL client GUI screen before leaving the computer. The ToL guest user may then execute the call normally. According to a first embodiment of the invention, the ToL client locks the user into the ToL client screen. Keystrokes and mouse cursor movements which would allow exiting the ToL client are prevented. According to a second embodiment, of the invention, the ToL client screen is "maximized" and the minimize or resize window functions are blocked. When the terminal subscriber returns, a password is entered to regain full access to the computer.