Call_ID        Start_Time
93856          2011-08-04 09:59:47.000
58796          2011-08-05 14:54:37.000
25489          2011-08-09 15:32:13.000

Call_ID        Start_Time                    Interval
93856          2011-08-04 09:59:47.000       0930
58796          2011-08-05 14:54:37.000       1430
25489          2011-08-09 15:32:13.000       1530

Select  Call_ID , Start_Time,
CASE WHEN DATEPART(minute,Start_Time)>30 THEN
RIGHT('0' + CAST(DATEPART(HOUR,Start_Time) AS VARCHAR),2) + '30'
ELSE
RIGHT('0' + CAST(DATEPART(HOUR,Start_Time) AS VARCHAR),2) + '00'
END
From Timezone
Group By Call_ID , Start_Time,
CASE WHEN DATEPART(minute,Start_Time)>30 THEN
RIGHT('0' + CAST(DATEPART(HOUR,Start_Time) AS VARCHAR),2) + '30'
ELSE
RIGHT('0' + CAST(DATEPART(HOUR,Start_Time) AS VARCHAR),2) + '00'
END

select Call_ID,
       Start_Time,
       right(100+datepart(hour, Start_Time), 2)+
       right(100+30*(datepart(minute, Start_Time)/30), 2) as Interval
from TimeZone

;WITH intervals(h) AS
(
  SELECT TOP (48) CONVERT(TIME(0), DATEADD(MINUTE, 30*(number), '00:00'))
    FROM master..spt_values
    WHERE number >= 0
    GROUP BY number
    ORDER BY number
)
SELECT
  t.Call_ID,
  t.Start_Time,
  Interval = REPLACE(CONVERT(VARCHAR(5), i.h), ':', '')
FROM intervals AS i
INNER JOIN dbo.TimeZone AS t
ON DATEDIFF(MINUTE, i.h, CONVERT(TIME(0), t.Start_Time)) BETWEEN 1 AND 30;