The problem with using "business days" as a definition is that every one has got a different idea of what are holidays. (government, bank, your particular business)
The easiest solution that I can think of, unfortunately, is to create a separate table. I would call this table tblDaysOff and it would have one field, Date. In this table I would put all dates for weekend days and any holidays that you might have.
Then, in your query, you would take the difference between the call received date and resolution date. From this number, you would subtract the number of non-work days between the two dates.
An example:
[tt]
SELECT DateDiff("d",ReceivedDate,ResolvedDate)-(
SELECT COUNT(*)
FROM tblDaysOff
WHERE [Date] Between ReceivedDate And ResolvedDate
)
FROM tblCallLog
[/tt]