Only functions and extended stored procedures can be executed from within a function.

Hi, l've created an function [GSM].[KPIAging], and test it in studio by substitule declare value, i.e.
DECLARE @sCellName VARCHAR(8)
DECLARE @dDate DATETIME
SET @sCellName = "CELL1M_1"
SET @dDate = CAST('06/Jun/2006' AS DATETIME)

EXEC GSM.KPIAging @sCellName, 'CSSR', @dDate

It work fine and return the desired result, but when l used this function in SQL,
SELECT DATEKEY, CELLREGIONKEY, CELL_NAME, CELL_ID, CSSR, GSM.KPIAging(Cell_Name, 'CSSR', @dDate)
  FROM GSM.GSMCellDaily_vw
 WHERE CSSR BETWEEN 0 AND 85
AND FULLDATE = @dDate
AND CM_SERV > 30
AND (TCH_TRAFFIC > 2 AND TCH_SEIZURES_ATTS > 30)

I got the following error, i.e.
Msg 557, Level 16, State 2, Line 19
Only functions and extended stored procedures can be executed from within a function.

Does anyone have any idea on this, and what's the workaround for this?

Thanks you!