This page provides information on using the DCI Blue wire to activate a "Cell Phone" or "ATT" function on your head unit.  On some head units (like Alpine) the ATT function operates differently than the MUTE function and many owners prefer it.

Most head units come with a MUTE function that can be activated either by a front panel button or the remote.  When activated it lowers the volume significantly, and in some cases may turn the volume completely off.  Many head units also incorporate an "ATT lead" or "Cell Phone" wire designed to be connected to a cell phone system to automatically shut off the volume when the phone is in use.

In most head units the MUTE and ATT functions are identical so there is no need to use the hard-wired ATT feature.  But in Alpine head units ATT acts differently than MUTE.  In MUTE mode the volume is just reduced, whereas ATT will turn the volume OFF and will pause CD playback, a nice feature. 

The way to set up the DCI for ATT operation is to enable the hard-wired mute signal and connect the DCI blue wire to the HU ATT lead.  Setting switch 3 to OFF (up) tells the DCI to generate a signal on the Blue wire instead of sending a mute command like it normally would.  Once thsi is done, when you press the dash mute button the DCI will activate the ATT function via the blue wire.  The HU volume will drop to zero, and it will stay at zero till you un-mute it via the dash mute button.  And CD (or MP3) playback will pause.  Cool.