Function Can't Be Found, But Don't Know Why

Hello,

I'm working on a function that will return the results in a SELECT statement.  But it's not being called properly, and I can't understand why it isn't working.  If anyone can help me out I'd greatly appreciate it.

Here's the function code:
1    FUNCTION [dbo].[UDFproduct_Search]
2    (
3        @search_string varchar(255)
4    )
5    RETURNS TABLE
6    AS
7    RETURN
8    (
9        SELECT product_id
10       FROM product
11       WHERE product_item_number LIKE '%' + @search_string + '%' OR
12             product_part_number LIKE '%' + @search_string + '%' OR
13             product_name LIKE '%' + @search_string + '%' OR
14             product_commodity LIKE '%' + @search_string + '%'
15   )

This SELECT statement returns the expected value:
SELECT * FROM UDFproduct_Search('CABINET')

This SELECT statement returns the expected value:
1    SELECT * FROM where_used WHERE product_id
2    IN(SELECT product_id FROM product
3       WHERE product_item_number LIKE '%' + 'CABINET' + '%' OR
4             product_part_number LIKE '%' + 'CABINET' + '%' OR
5             product_name LIKE '%' + 'CABINET' + '%' OR
6             product_commodity LIKE '%' + 'CABINET' + '%')

This SELECT statement returns errors:
SELECT * FROM where_used WHERE product_id IN(UDFproduct_Search('CABINET')
Error:
'UDFproduct_Search' is not a recognized built-in function name.

I've tried adding dbo to the function call, and in various locations, but I still get the error.  Hope someone can point something out I missed.

Thanks.