Problem: Flash Fill is not working with numbers. Actually, I see the grey numbers appear, but then Excel erases them.
Strategy: Chad Rothschiller at Microsoft points out there are too many coincidences that can happen in numbers. With only 10 digits, Chad might think he knows the pattern but then screw up your data. So, Chad shows you the preview, but then withdraws it. It is up to you to go to the blank next blank cell and press Ctrl+E.