how to use function-based indexes?

Hello.
I'm running Oracle 9.0.1.2 but I cannot use function-based indexes.

SQL> CREATE INDEX XIE1ACQUIRENTI ON ACQUIRENTI   2 (
  3 UPPER(ACQUIRENTE_RAG_SOC)
  4 );

Index created.

SQL> explain plan for
  2 select * from acquirenti
  3 where upper(acquirente_rag_soc) like '%VERO%' ;

Explained.

SQL> select * from plan_view;

OPERATION OPTIONS OBJECT_NAME POSITION

-------------------- -------------------- --------------------
----------
SELECT STATEMENT                                                       
1
  TABLE ACCESS       FULL                 ACQUIRENTI                   

Where is my error?

TIA, Cris

-- 
Cristian Veronesi ><((((º> http://www.crpa.it

There are no good wars, with the following exceptions: The American 
Revolution, World War II, and the Star Wars Trilogy. (Bart Simpson)