use phone_tapping

-- Find calls from Alex
db.calls.find({"from.first_name":"Alex"})

-- Find the phone number of Alex at this time
db.users.find({"first_name":"Alex", "from":{$lt:ISODate("2018-02-26T11:45:23Z")}, "to":{$gt:ISODate("2018-02-26T11:45:23Z")}})