SQL & PL/SQL

Dear Gurus,

I have below table CDR
Name Null Type
------------------------------ -------------------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
STARTTIME NOT NULL DATE
SUBSCRIBERNUMBER NOT NULL NUMBER
CALLINGNUMBER NOT NULL VARCHAR2(20)
CALLEDNUMBER NOT NULL VARCHAR2(20)

I am regularly firing below query

SELECT count(*)
FROM CDR data
WHERE STARTTIME BETWEEN '01-Jul-2009 00:00:00' and '31-May-2012 23:59:59'
AND REGEXP_LIKE(data.SUBSCRIBERNUMBER, '^98721[0-9]*[5]+[0-9]*$');

since there is REGEXP_LIKE is being used, Can I use function based index to improve performance.

Thanking in advance
Sanjeev