I'm thinking that you might want to include "business hours" on weekends. The easiest way to do it would be using DATEPART. However, you might encounter with the problem of holidays and there's where a calendar table will help you.
You would end up with something like this depending on your calendar table.
Select sum(abncalls)
from TABLE t
JOIN CalendarTable c ON t.date = c.date
where date between @StartDate) and (@EndDate)
and (starttime not in (800,830,900,930,1000,1030,1100,1130,1200,1230,1300,1330,1400,1430,1500,1530,1600,1630)
or is_workday = 0)