db={"call":[{"_id":1,"user":123}],"contact":[{"_id":1,data:{key1:123,key2:555}}]}

db.call.aggregate([{"$lookup":{from:"contact",let:{"mobile":"$user"},pipeline:[{"$project":{d:{$objectToArray:"$data"},doc:"$$ROOT",}},{"$unwind":{path:"$d",preserveNullAndEmptyArrays:true}},{"$match":{$expr:{$eq:["$$mobile","$d.v"]}}},],as:"name"}}])