Queries :: Limit Query Results To Numeric Value Of Text Field?

I have a table that has several fields including CallID (autonumber) and SKU (text)

SKU can be anything up to 9 characters, sometimes numeric sometimes alphanumeric. For example: 24300, AA23145, G58d444, 24999, 89332,...

Based on the Count of CallID I can easily get the top20 calls on each SKU. This is the query I use for that:

Code:
SELECT TOP 20 Count(Calls.CallID) AS CountOfCallID, Calls.SKU
FROM Calls
GROUP BY Calls.SKU
HAVING ((Not (Calls.SKU) Is Null))
ORDER BY Count(Calls.CallID) DESC;

The problem is that now I have been asked to create two different lists. One that has the top 20 SKU that range from 24520 and 24599 and another one that does the res tof the SKUs.

Obviously my problem is that the SKU field is text, not numbers so I can't just limit the results in the query by using "Between 24520 and 24500" in the query criteria.