Mitel console buttons are prone to be stuck down sideways. This is usually caused by dust or dirt around the button. If this happens the console appears to look normal, but it will not respond to any buttons that are pressed. It is hard to see which button has become wedged sideways. The best way to locate the problem is to wipe the palm of your hand over all the keys and you will feel the one that does not feel right. When you locate the sideways button, simply press down on the opposite end and it will pop back up. Do not use any sharp object on the plastic surface, it will be damaged.
To clean the buttons - remove the clear plastic overlay and the paper underlay. Spray a light amount of WD40 and wipe clean with paper napkin. Wait for at least 24 hours for the WD40 to dry, then reinstall the paper and the plastic. Do not use any type of harsh cleaning fluid such as Windex. Its acid is sure to destroy the components inside.