Database
[{"_id":ObjectId("5e8f67091aa1cc3d2158ade0"),"name":"UAE","callingCode":971.0},{"_id":ObjectId("5e8f67091aa1cc3d2158ade1"),"name":"USA","callingCode":1.0},{"_id":ObjectId("5e8f67091aa1cc3d2158ade2"),"name":"UK","callingCode":44.0}]
Query
db.collection.aggregate([{$match:{$expr:{$eq:[{$let:{vars:{value:{$regexFind:{input:"971524500000",regex:{$toString:"$callingCode"}}}},in:"$$value.idx",}},0]}}}])