D600 Port Replicator Modem Issue

In our company, we have several port replicators for the Latitude D600 Laptops.  For some reason when the laptop is actually docked and connected to the port replicator, the modem on the port itself does not work.  When we connect the phone cord to the actual laptop, it works.  Can someone give any suggestions as to why this is happening and if there are any solutions to this problem?