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2-line 3-way jack.
Home Depot also carries a comparable RCA model.
Important Note: The trick is that you must plug this adapter into the modem itself not into the wall jack. Plug one end of the phone cord into the port marked "L2" on the adapter and then plug the other end into the wall jack.
Alternative Method: If a line swapper is not available you can rewire the wall plate to swap the inner and outer pairs. Reference the Line Swap Schematic for details.