WITH DigitsCTE(digit) AS
(
   SELECT 0 UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL
   SELECT 5 UNION ALL SELECT 6 UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9
)
, NumbersCTE(number) AS
(
   SELECT ROW_NUMBER() OVER(ORDER BY(SELECT NULL)) - 1
     FROM DigitsCTE AS D0
          CROSS JOIN
          DigitsCTE AS D1
)
, MinutesCTE(minute) AS
(
   SELECT TOP(60) number
     FROM NumbersCTE
)
, HoursCTE(hour) AS
(
   SELECT TOP(24) number
     FROM NumbersCTE
)
, OneDayMinutesCTE(minute) AS
(
   SELECT hour * 60 + minute
     FROM HoursCTE AS H
          CROSS JOIN
          MinutesCTE AS M
)
, PhoneLogDaysCTE AS
(
   SELECT date
     FROM (SELECT DATEADD(DAY, DATEDIFF(DAY, '20100101', StartTime), '20100101')
             FROM dbo.PHONE_LOG

            UNION ALL

           SELECT DATEADD(DAY, DATEDIFF(DAY, '20100101', StopTime), '20100101')
             FROM dbo.PHONE_LOG) AS T(date)
    GROUP BY date
)
, PhoneLogVoidsCTE(void_date, group_identifier) AS
(
   SELECT D.date, DATEADD(MINUTE, -1 * ROW_NUMBER() OVER(ORDER BY D.date), D.date)
     FROM (SELECT DATEADD(MINUTE, ODM.minute, PLD.date)
             FROM PhoneLogDaysCTE AS PLD
                  CROSS JOIN
                  OneDayMinutesCTE AS ODM) AS D(date)
    WHERE NOT EXISTS(SELECT *
                       FROM PHONE_LOG AS PL
                      WHERE PL.StartTime <= D.date
                        AND PL.StopTime >= D.date)
)
SELECT MIN(PLV.void_date) AS start_time, MAX(PLV.void_date) AS end_time
  FROM PhoneLogVoidsCTE AS PLV
 GROUP BY PLV.group_identifier
 ORDER BY start_time, end_time;