[{"_id":"id1","contacts":[{"phone":"xxx","email":"id1@gmail.com"},{"phone":"yyy"},{"phone":"zzz","email":"id1_1@gmail.com"}]},{"_id":"id2","contacts":[{"phone":"aaa","email":"id2@gmail.com"},{"phone":"bbb"},{"phone":"ccc"}]}]

db.collection.aggregate([{"$project":{"contacts":"$contacts.phone"}}])