[{store:"Jacks Pizza",storeNumbers:[{"chef":"Giovanni","number":"7203305544"}]},{store:"Felicias Kitchen",storeNumbers:[{"chef":"Gina","number":"+19161214594"}]}]

db.collection.update({"storeNumbers.number":{$exists:true,$ne:"",$not:{$regex:"^\\+1"}}},[{$set:{storeNumbers:{$map:{input:"$storeNumbers",in:{$mergeObjects:["$$this",{$cond:[{$and:[{$not:{$regexMatch:{input:"$$this.number",regex:"^\\+1"}}},{$ne:["$$this.number",""]},{$eq:[{$type:"$$this.number"},"string"]}]},{number:{$concat:["+1","$$this.number"]}},{}]}]}}}}}])