DSS mean Direct Station Select---those are the buttons on the left side of your phone. If station 21 goes off hook, the first key (upper left) will light. To intercom station 21--push that key. To intercom station 24 push the 4th key from the top left going down on the left---1,2,3,4 etc to 8----the 9 or station 29) is the top button on the right in the 16 button section.