Assumptions:
both worksheets are in the same spreadsheet
worksheets are named State and Phone
State: Column A contains the area code, Column B contains the State, for this to work I had to convert the area codes in column A to text. It didn't work before that.
Phone: the formula "=left(B1,3)" is in cell D1
My formula was =VLOOKUP(D1,State!A2:B51,2)
If it gives you trouble keep playing with the parameters. The value you are comparing to has to be the same data type as is in the table. You can fake that by inserting a column to convert the area codes to text.