Grouping Question

I have a table defined as

CREATE TABLE mytable
(
   carrier varchar,
   start_time timestamp with time zone,
   call_date date,
   cost numeric,
   call_length numeric
)

I want to create a query that will generate a the following columns:

carrier, week, sum(call_length) as totallength, sum(cost) as total_cost
from mytable
group by carrier, (WHAT HERE?)
order by week, carrier


week is defined as a date range so something like 7/6/2009 - 7/13/2009

I would need the timestamps to be grouped into 7 day intervals
starting from the first one and moving through the table. is this
possible in a single query or would I have to write a function ?