Sector 3556 is an area of space in the Delta Quadrant. The terminus of the Barzan wormhole shifts between Sector 3556 in the Delta Quadrant and a point beyond the Denkiri Arm in the Gamma Quadrant.
In 2366 the Ferengi Arridor and Kol were stranded in Sector 3556 when they travelled through the wormhole but did not re-enter it before the terminus shifted again. (TNG: "The Price")