[{id:1,phone:"+15555555555",name:"Acme CO.",vendorcode:"ACMEC"},{id:2,phone:"+15555555555",name:"Acme corporation company",vendorcode:"ACMECOMPANY"},{id:3,phone:"+15555555555",name:"Acme Incorporated",vendorcode:null}]

db.collection.aggregate([{$group:{_id:"$phone",phone:{$first:"$phone"},name:{$first:"$name"},vendorcode:{$addToSet:{$ifNull:["$vendorcode","$$REMOVE"]}}}},{$project:{_id:0}}])