SELECT COUNT(servedmsisdn) as qt_voice_subscribers
FROM
(SELECT distinct servedmsisdn
FROM top_moc 
WHERE date between '20070501' and '20070530' AND qt_moc>0
UNION
SELECT distinct servedmsisdn
FROM top_moc 
WHERE date between '20070501' and '20070530' AND qt_mtc>0
UNION
SELECT distinct servedmsisdn
FROM top_moc 
WHERE date between '20070501' and '20070530' AND qt_cfw>0)

AND HAVE COVER INDEXES: ON (date,servedmsisdn,qt_moc) ON (date,servedmsisdn,qt_mtc) ON (date,servedmsisdn,qt_cfw)