select
col1,
DECODE((decode(col2,'Y',col3,NULL)),NULL,
(SELECT PHONE_NUMBER FROM PHONES
WHERE CUSTOMER_ID = A.COL1 AND ROWNUM<=1)
,COL3) PHNO
from
(
SELECT phon.customer_id col1,phon.number col2,phone_number col3
FROM phones phon, customers cust
WHERE phon.customer_id (+)= cust.customer_id
) A