UDF As Computed Column In Temporary Table In Proc

I've created a stored procedure that creates and uses a temporary table with a computed column. I want the computed column to call a user-defined function(UDF) that I've created. Here's an example:

CREATE PROCEDURE [dbo].[usp_Proc]
(
@Date datetime
)
AS
BEGIN


--Drop the temp table if it still exists so reports come out accurate
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[#temp]') AND type in (N'U'))
DROP TABLE #temp;

--Create the temp table for use in this stored procedure
IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[#temp]') AND type in (N'U'))
CREATE TABLE #temp (
[ID] INT PRIMARY KEY IDENTITY(1,1),
[Column1] NVARCHAR (30) DEFAULT ('XXXX-XXXXX'),
[Column2] INT DEFAULT (0),
[Column3] INT DEFAULT (0),
[Column4] INT DEFAULT (0),
[Column5] as ([Column2] + [Column3] + [Column4]),
[Column6] as (dbo.FunctionName('5381', [Column1]))
)

--Insert data
INSERT INTO #temp([Column1], [Column2], [Column3], [Column4])
SELECT 'String', 1, 2, 3

--Perform more calculations
<snipped...>

SELECT * FROM #temp

DROP TABLE #temp

END

This is an example of the function:

CREATE FUNCTION [dbo].[FunctionName]
(
-- Add the parameters for the function here
@Type nvarchar(4),
@Quantity int
)
RETURNS Money
AS
BEGIN

RETURN (cast((SELECT ([Value] * cast(@Quantity as int)) FROM tblTable WHERE [ID] = @Type) as Money))
END

The error message I'm getting after I've created both the stored procedure and the UDF is when calling the stored procedure. Here it is:

Msg 4121, Level 16, State 1, Procedure usp_Proc, Line 13
Cannot find either column "dbo" or the user-defined function or aggregate "dbo.FunctionName", or the name is ambiguous.

There's no way the function name is ambiguous. Is it even possible to do what I'm trying to do or am I just calling it the wrong way?

Hosmerica