This means one of five things and almost never a sign of a defect:
1) The compatibility switch is set to the wrong letter (the switch labeled A thru I. A and G are the letters that work with the most phones).
2) The handset cord and connector cord are plugged into the wrong jacks on the back of the amp and need to be reversed (the connector cord must be plugged into the jack labeled with the picture of the entire phone or the amp will not work. The handset, however, will work even if it is plugged into the wrong jack).
3) The batteries are in backwards.
4) The batteries are dead. This is the most common issue. You should always try changing these (and then double check to make sure the batteries are not backwards) even if the customer has already changed them once.
5) The connector cord is not plugged all the way in. Neither the headset or handset will work if this is the case. There is also a possibility the connector cord is bad (but this is incredibly rare). Customer may also be trying to use a cord that did not come with the unit.