Sorry, I was in another mode when i wrote that. disregard it. You need to leave out the left and right and use substring. You would use qwertyguy's solution if you alread created a table called tbl.
Do like this:
UPDATE dbo.tbl
SET AC = MID(RTRIM(LTRIM([Phone_Number])),1,3),
Number = MID(RTRIM(LTRIM([Phone_Number])), 4, 7)
Sorry about goof above.