works:
select cdr.src from cdr where cdr.src not in 
(select callerid.cid_number from callerid)
order by cdr.src;

works:
select cdr.dst from cdr where cdr.dst not in 
(select callerid.cid_number from callerid)
order by cdr.dst;


syntactically correct, but doesn't seem to be returning correct results:
select concat(cdr.src, "/", cdr.dst) as callrec from cdr
where cdr.src not in (select callerid.cid_number from callerid)
and cdr.dst not in (select callerid.cid_number from callerid)
order by callrec;