Issue with ROW_NUMBER and DISTINCT
DISTINCT is evaluated last or almost last of all the clauses. Therefore, it can't be used on the same level as ROW_NUMBER to get the results you want. Either use a derived table with DISTINCT:

SELECT
  Dept
FROM (
  SELECT
    Dept,
    ROW_NUMBER() OVER (ORDER BY Dept) AS RowID
  FROM (
    SELECT DISTINCT
      Dept
    FROM Department
    WHERE Dept ... 
  ) AS d
) AS Calls
WHERE RowID ...
;


or replace DISTINCT with GROUP BY:

SELECT
  Dept
FROM (
  SELECT
    Dept,
    ROW_NUMBER() OVER (ORDER BY Dept) AS RowID
  FROM Department
  WHERE Dept ...
  GROUP BY Dept
) AS Calls
WHERE RowID ...
;