Task_Helper.cls

public class Task_Helper {

    public static Map<String, Schema.SObjectField> FIELD_MAP = Schema.SObjectType.Task.fields.getMap();

    public static void afterInsert(List<Task> newList){

        Map<Id, Datetime> dateMap = new Map<Id, Datetime>();
        Map<Id, String> descriptionMap = new Map<Id, String>();
        Map<Id, Id> userMap = new Map<Id, Id>();
        Map<String, Integer> sumMap = new Map<String, Integer>();
        List<Lead> leadList = new List<Lead>();

        for (Task t : newList){
          if (t.WhoId != null && String.valueOf(t.WhoId.getSObjectType()) == 'Lead' && t.Subject.substring(0,4).toLowerCase() == 'call'){
            if (!dateMap.containsKey(t.WhoId)){
                dateMap.put(t.WhoId, t.CreatedDate);
                descriptionMap.put(t.WhoId, t.CallDisposition);
                userMap.put(t.WhoId, t.OwnerId);
            } else {
              // check that this current task's date might be greater than the one already on the map
              if (t.CreatedDate > dateMap.get(t.WhoId)){
                dateMap.put(t.WhoId, t.CreatedDate);
                descriptionMap.put(t.WhoId, t.CallDisposition);
                userMap.put(t.WhoId, t.OwnerId);
              }
            }
          }
        } // closes for loop through Task


        for (AggregateResult ar : [SELECT WhoId, count(Id) cnt FROM Task WHERE WhoId IN :dateMap.keySet() AND Subject LIKE 'Call%' GROUP BY WhoId]){
          sumMap.put(String.valueOf(ar.get('WhoId')), Integer.valueOf(ar.get('cnt')));
        }

        for (Lead l : [SELECT Id,Last_Call_Disposition__c,Last_Call_Made__c,Last_Call_Made_By__c FROM Lead WHERE Id IN :dateMap.keySet()]){
          if (l.Last_Call_Made__c != null){
            if (l.Last_Call_Made__c < dateMap.get(l.Id)){
              l.Last_Call_Made__c = dateMap.get(l.Id);
              l.Last_Call_Disposition__c = descriptionMap.get(l.Id);
              l.Last_Call_Made_By__c = userMap.get(l.Id);
              l.Total_Amount_Of_Calls_Made__c = sumMap.get(l.Id);
              leadList.add(l);
            }
          } else {
            l.Last_Call_Made__c = dateMap.get(l.Id);
            l.Last_Call_Disposition__c = descriptionMap.get(l.Id);
            l.Last_Call_Made_By__c = userMap.get(l.Id);
            l.Total_Amount_Of_Calls_Made__c = sumMap.get(l.Id);
            leadList.add(l);
          }
        }

        update leadList;
        System.debug(leadList);

    }

}