Calculating the AVG value per GROUP in the GROUP BY Clause

I'm working on a query in SQL Server 2005 that looks at a table of recorded phone calls, groups them by the hour of the day, and computes the average wait time for each hour in the day.

I have a query that I think works, but I'm having trouble convincing myself it's right.

SELECT
    DATEPART(HOUR, CallTime) AS Hour,
    (AVG(calls.WaitDuration) / 60) AS WaitingTimesInMinutes
FROM (
    SELECT
        CallTime,
        WaitDuration
    FROM Calls
    WHERE DATEADD(day, DATEDIFF(Day, 0, CallTime), 0) = DATEADD(day, DATEDIFF(Day, 0, GETDATE()), 0)
        AND DATEPART(HOUR, CallTime) BETWEEN 6 AND 18
) AS calls
GROUP BY DATEPART(HOUR, CallTime)
ORDER BY DATEPART(HOUR, CallTime);

To clarify what I think is happening, this query looks at all calls made on the same day as today, and where the hour of the call is between 6 and 18 -- the times are recorded and SELECTed in 24-hour time, so this between hours is to get calls between 6am and 6pm.

Then, the outer query computes the average of the WaitDuration column (and converts seconds to minutes) and then groups each average by the hour.

What I'm uncertain of is this: Are the reported BY HOUR averages only for the calls made in that hour's timeframe? Or does it compute each reported average using all the calls made on the day and between the hours? I know the AVG function has a optional OVER/PARTITION clause, and it's been a while since I used the AVG group function. What I would like is that each result grouped by an hour shows ONLY the average wait time for that specific hour of the day.

Thanks for your time in this.