2degrees: We can't open up every single number available. We've got to keep some aside for future business growth and our testing purposes. These need to be set aside in blocks so we've kept aside the 7, 8, 9 ranges for these purposes.
As posted above if you're after something out of those ranges it is definitely not available.
I find that the best way to search is to enter in the full number you want and if it says information not available then it means that this number has been taken.
If you're just trying to find something you fancy - nothing specific - then try with a number combination ie 4444 or 123 and you'll get a better feel for what number ranges that have quadruple 4's or 123's are available.