Nested select

For this table:

START_TIME DATE
CHANNEL NUMBER(3)
CALL_TOT NUMBER
DUR_TOT NUMBER

… I use the query:

select to_char(start_time,’dd-MON-yy hh:mi:ss AM’), sum(call_tot)
from trasum
group by to_char(start_time,’dd-MON-yy hh:mi:ss AM’);

… to Sum the call totals of all channels with respect to each hour of the day (minutes and seconds aren’t counted).

Now, I want to see the Maximum call total for each day and the hour in which it falls. How mightI do that?