Need to optimize SQL statement

I have a log table with the following cols


ID is PK and MobileNo is Varchar(10)

I use the following statement to get the count of new added mobile numbers (numbers does not exists before a specific date)

SELECT count(MobileNo) From MyTable
where LOG_TIME >= '2010-12-01'
and LOG_TIME < '2010-12-05'
and MobileNo NOT IN (select MobileNo from MyTable where LOG_TIME < '2010-12-01' )

It takes very long time to execute and I need to use a better (optimized) query

The table has about 6 million rows

Try creating an index on mobilenr.

There is an index on MobileNo but the query is still very slow