If the phone system has 6 phones working on it already means there are 8 DLI ports available. Like PVJ says, the DLI cards are identified by the color light that is lit up on the uni-card. I would check and see if the P1-P4 are used on the processor, that can also carry a DLM card. Look at where the other card is, looking for the second p? with red light. Then you can plug it into there.