Usually both 4 core and 6 core circuits are acceptable for 10/100BT, and 8 core and 12 core circuits are acceptable for 10/100/1000BT PHYs. Also, there may be only one ICM vendor on the reference design, but other manufacturers product may work. Consult the application notes or magnetics guide from the PHY manufacturer again for guidance. After the type of circuit is selected, choose a mechanical configuration that best meets the design’s needs. The link shown here will lead you to the Stewart supplier page at DigiKey, from there you can view the link to the PHY to MagJack cross reference document via the link located under the Tools and Support tab.