You are filtering on left 3 chars of PostalCode (a.k.a. zip code), I think you meant to use left 3 chars of Phone Number instead since that is where you find the area code.
Also, you have created a scalar function, so after you change to filter on area code instead of postal code, and if there is more than one customer with the 706 area code, you will likely get the following error:
"Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression."
If you are wanting to retrieve all customers that have a certain area code, then you will need a table value function. See http://technet.microsoft.com/en-us/library/ms189294(v=sql.105).aspx
If you do not want duplicate customer names returned, then add DISTINCT after the SELECT in your function.
Note that when selecting from a table valued function, you should use the syntax
SELECT Customers FROM dbo.ufnGetCustomerByAreaCode('706')
instead of
SELECT dbo.ufnGetCustomerByAreaCode('706')