Case expressions may only be nested to level 10

I have the following code for handling phone numbers such as country code for Australia +61, 61, 001161 etc. The problem that I have I can't insert any CASE statement anymore under: CASE WHEN LEFT(@BPartyNo, 4) = '+610'

It said that Case expressions may only be nested to level 10

How do I streamline this TSQL so I can put more CASE?

USE [TelcoStage_PROD]
GO
/****** Object:  UserDefinedFunction [dbo].[ufn_stg_ProperBPartyNoExtra]    Script Date: 07/12/2010 15:27:52 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

--=====================================================================================================================
-- OBJECT NAME          : dbo.ufn_stg_ProperBPartyNoExtra
-- INPUTS               : @BPartyNo 
-- OUTPUTS              : VARCHAR(32)
-- RETURN CODES         : N/A
-- DEPENDENCIES         : N/A
-- DESCRIPTION          : This function is used to get the extra after 10 character (MNET or S)
--
-- EXAMPLES (optional)  : N/A
--
-- HISTORY:
-- #-----------------------------------------------------------------------------------------------------------------
-- # DATE       | VERSION        | MODIFIED BY | DESCRIPTION
-- #-----------------------------------------------------------------------------------------------------------------
====================================================================================================================


ALTER FUNCTION [dbo].[ufn_stg_ProperBPartyNoExtra](@BPartyNo AS VARCHAR(MAX))RETURNS VARCHAR(32)
AS
BEGIN   
    DECLARE @Return VARCHAR(32);

    SET @Return = '';

    IF (LEN(@BPartyNo) > 0) 
        SELECT @Return = CASE WHEN LEFT(@BPartyNo, 4) = '+610' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 5, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 5, LEN(@BPartyNo)) ) ) ELSE
                                    CASE WHEN LEFT(@BPartyNo, 3) = '+61' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 4, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 4, LEN(@BPartyNo)) ) ) ELSE
                                        CASE WHEN LEFT(@BPartyNo, 2) = '61' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 3, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 3, LEN(@BPartyNo)) ) ) ELSE       
                                            CASE WHEN LEFT(@BPartyNo, 6) = '001161' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 7, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 7, LEN(@BPartyNo)) ) ) ELSE 
                                                CASE WHEN ( LEFT(@BPartyNo,2) = '01' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo)) ELSE 
                                                    CASE WHEN ( LEFT(@BPartyNo,2) = '02' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo)) ELSE 
                                                        CASE WHEN ( LEFT(@BPartyNo,2) = '03' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo)) ELSE 
                                                            CASE WHEN ( LEFT(@BPartyNo,2) = '04' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo)) ELSE 
                                                                CASE WHEN ( LEFT(@BPartyNo,2) = '07' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo)) ELSE 
                                                                    CASE WHEN ( LEFT(@BPartyNo,2) = '08' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo)) ELSE '' END
                                                                END
                                                            END
                                                        END
                                                    END
                                                END
                                            END
                                        END
                                    END
                        END;
    ELSE
        SELECT @Return = '';

    RETURN @Return
END

Solution

They don't need to be nested at all:

   SELECT @Return = CASE WHEN LEFT(@BPartyNo, 4) = '+610' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 5, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 5, LEN(@BPartyNo)) ) )
                         WHEN LEFT(@BPartyNo, 3) = '+61' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 4, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 4, LEN(@BPartyNo)) ) )
                         WHEN LEFT(@BPartyNo, 2) = '61' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 3, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 3, LEN(@BPartyNo)) ) )
                         WHEN LEFT(@BPartyNo, 6) = '001161' THEN SUBSTRING('0' + SUBSTRING(@BPartyNo, 7, LEN(@BPartyNo)), 11, LEN( '0' + SUBSTRING(@BPartyNo, 7, LEN(@BPartyNo)) ) )
                         WHEN ( LEFT(@BPartyNo,2) = '01' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo))
                         WHEN ( LEFT(@BPartyNo,2) = '02' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo))
                         WHEN ( LEFT(@BPartyNo,2) = '03' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo))
                         WHEN ( LEFT(@BPartyNo,2) = '04' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo))
                         WHEN ( LEFT(@BPartyNo,2) = '07' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo))
                         WHEN ( LEFT(@BPartyNo,2) = '08' AND LEN(@BPartyNo) > 10 ) THEN SUBSTRING(@BPartyNo, 11, LEN(@BPartyNo))
                         ELSE ''
                     END

Personally, I would restructure your code so that you make sure the data in sanitized upon input, rather than trying to sanitize it now (when it's clearly too late...). Or at least do the conversion in your client language (i.e. in whatever is calling this sproc), which is hopefully more suited to the task of string manipulation than T-SQL is.