First, make sure it's wired correctly. The green wire coming off of the sled wiring harness is the cell phone mute, and should be connected to the corresponding wire on your cell phone kit.

Next, make sure there's not a conflict with the serial port plug in the docking sled. The cell phone mute wire shares a pin with the serial port plug. So if you're using the serial port plug for something else such as a GPS, you might want to make sure that the corresponding wire is disconnected from the serial plug.

Then, open the "Configure Player" screen in Emplode and select the "Mute" tab to control how the player should behave when the cell phone mute is on. The behavior will depend on whether your phone kit's mute line goes high or goes low when it's off-hook, you need to tell it which kind of phone mute you have. If you don't know, you can test your phone kit with a multimeter.

Note that some phone kits, instead of switching between 0v and 12v, will switch between grounded (0v) and open-circuit. In that case, the 0v/12v sensing won't work and you'll need to make a modification to the wiring in order to make it work. Attach a 10k ohm resistor between +12v and the mute wire, similar to the power antenna solution described here. After that, the 0v/12v sensing should work.