SQL> -- wrong way:
SQL> SELECT TO_DATE ('19-NOV-2002 12:01:15', 'DD-MON-YYYY') FROM DUAL
  2  /
SELECT TO_DATE ('19-NOV-2002 12:01:15', 'DD-MON-YYYY') FROM DUAL
                *
ERROR at line 1:
ORA-01830: date format picture ends before converting entire input string

SQL> 
SQL> 
SQL> -- correct way:
SQL> SELECT TO_DATE ('19-NOV-2002 12:01:15', 'DD-MON-YYYY HH24:MI:SS') FROM DUAL
  2  /

TO_DATE('
---------
19-NOV-02

-- if DateColumnName is VARCHAR2:
SELECT   to_char(to_date( DateColumnName ,'DD-MON-YYYY HH24:MI:SS'),'dd/mm/yyyy'), 
         sum(CAllsReceived) 
from     VMCC_STATISTICS 
where    time_from between to_date('30/01/2003' , 'dd/mm/yyyy') 
                   AND     (to_date('30/01/2003' , 'dd/mm/yyyy') + 4) 
group by to_date(time_from , 'dd/mm/yy') 
order by to_date(time_from , 'dd/mm/yy');

-- if DateColumnName is date or timestamp:
SELECT   to_char(DateColumnName, 'dd/mm/yyyy'), 
         sum(CAllsReceived) 
from     VMCC_STATISTICS 
where    time_from between to_date('30/01/2003' , 'dd/mm/yyyy') 
                   AND     (to_date('30/01/2003' , 'dd/mm/yyyy') + 4) 
group by to_date(time_from , 'dd/mm/yy') 
order by to_date(time_from , 'dd/mm/yy');