Something like

SELECT ;
  BoIdBase, SUM(CallTot) AS CallTot ;
  FROM Details ;
  GROUP BY  1 ;
  INTO CURSOR DetSum 

INDEX ON BoIdBase TAG BoIdBase
SET ORDER TO BoIdBase 

SELECT LineItems
SET RELATION TO BoIdBase INTO Details
REPLACE LineItems.CallTot WITH DetSum.CallTot ;
  FOR FOUND("DetSum")

>I have a table, LineItems, which is related into Details:
>I scan through LineItems, and for each related child record, I
>total up a field called CallAct:
>
>
>SELECT LineItems
>SCAN
>
>  nTotal = 0
>
>  SELECT Details
>  IF NOT EOF()
>    DO WHILE NOT EOF()
>      nTotal = nTotal + Details.CallAct
>    ENDDO
>  ENDIF
>
>  REPLACE IN LineItems CallTot WITH nTotal NEXT 1
>
>ENDSCAN
>
>
>Anyone know how I can do this using SQL?
>
>Many thanks