WITH Timestamps AS
(
  SELECT startdt AS ts FROM dbo.Phones
  UNION ALL SELECT COALESCE(enddt, 99991231) FROM dbo.Phones
  UNION ALL SELECT startdt FROM dbo.Addresses
  UNION SELECT COALESCE(enddt, 99991231) FROM dbo.Addresses
),
TSRN AS -- Timestamps with Row Numbers
(
  SELECT ts, ROW_NUMBER() OVER(ORDER BY ts) AS pos 
  FROM Timestamps
),
Intervals AS
(
  SELECT Cur.ts AS startdt, Nxt.ts AS enddt
  FROM TSRN AS Cur
    JOIN TSRN AS Nxt
      ON Nxt.pos = Cur.pos + 1
)
SELECT I.startdt, I.enddt, P.phone, A.address
FROM Intervals AS I
  LEFT OUTER JOIN dbo.Phones AS P
    ON COALESCE(P.enddt, 99991231) > I.startdt
    AND P.startdt < I.enddt
  LEFT OUTER JOIN dbo.Addresses AS A
    ON COALESCE(A.enddt, 99991231) > I.startdt
    AND A.startdt < I.enddt;