ISDN 10/20/30 is a primary rate digital service with a maximum capacity of 30 channels (call lines). ISDN2 is a basic rate service with a maximum capacity of 2 channels. Both services deliver digital phone lines with the main difference being capacity and the required PBX functionality. You need a PRI (primary rate interface) card for ISDN and a BRI (basic rate interface) card for ISDN2.