Return to Revisions

Something like this might work

SELECT 
  count(1)
  ,DATEADD(MINUTE,(DATEPART(MINUTE, cl_StartDate) / 15)*15,(DATEADD(HOUR, DATEDIFF(HOUR, 0, cl_StartDate), 0)))
FROM tbl_Calls
WHERE 
  cl_StartTime BETWEEN @StartDate AND @EndDate
GROUP BY 
  DATEADD(MINUTE,(DATEPART(MINUTE, cl_StartDate) / 15)*15,(DATEADD(HOUR, DATEDIFF(HOUR, 0, cl_StartDate), 0)))

To get the quarter this will give you a value of 0-3. Multiply by 15 to get 0,15,30,45.

DATEPART(MINUTE, date_recorded) / 15 

To round down cl_StartTime to a full hour.

DATEADD(HOUR, DATEDIFF(HOUR, 0, date_recorded), 0)

Then you just add it together like in the query to get the correct timestamp and group by that.