SELECT sum(billsec)/60 FROM cdr
join pre_numbers on cdr.dst = pre_numbers.pre_number
WHERE calldate >= '2014-03-31 00:00:00'
AND calldate <= '2014-04-23 23:59:59'
AND id NOT IN
(SELECT cdr_id FROM cdr_billed WHERE calldatetime >= UNIX_TIMESTAMP('2014-03-31 00:00:00') AND calldatetime <= UNIX_TIMESTAMP('2014-04-23 23:59:59'))
AND dst REGEXP '^-?[0-9]+$'
and pre_numbers.type = 1
;