(DISCONTINUED) The three lights on the S12 base are from L to R are: Headset indicator light - if it’s on, the headset is on Power indicator light - if it’s on, the S12 has power Mute indicator light - if it’s on, the user is muted Buttons and dials: On the front of the S12 base - The button on the left is the headset button - use this to access the call in the headset - The dial in the center is the incoming volume control - The button on the right is the mute button - use this to mute the user On the left of the S12 base - This is the Compatibility Dial (1 or 4 are usually the best settings) Here is a loose guide to compat: 1- most phones 2- Definity phones 3- Merlin Phones 4- GE 2-9450 phones/Cisco 7900 5- Siemens phones 6- NW Bell phones On the right side of the S12 base - The dial will fine-tune the outgoing volume - The two dots are the master outgoing volume selectors - the small one is quieter, the large one is louder