Hi all,
I written one trigger and test class, but code coverage is 33%. can any one help me

Trigger:-
---------
trigger LeadPreventUpdatePhone on Lead (before update) {         
    string UserName = userinfo.getUserName();
    if(UserName.left(4) == 'sfdc'){
    for (Lead Lead : System.Trigger.new) {   // This line is not covered the code
        Lead oldLead = Trigger.oldMap.get(Lead.ID); // This line is not covered the code
        if(Lead.Phone != oldLead.Phone){ // This line is not covered the code
            Lead.Phone = oldLead.Phone; // This line is not covered the code
        }
    }
    }


Test class:-
--------------
@isTest
public class LeadPreventUpdatePhoneTestClass
{

    @isTest static void LeadPreventUpdatePhone()
    {
       Profile p = [SELECT Id FROM Profile WHERE Name='SumT Support'];
       User u = new User(Alias = 'Test', Email='Test@testorg.com',
       EmailEncodingKey='UTF-8', LastName='Test', LanguageLocaleKey='en_US',
       LocaleSidKey='en_US', ProfileId = p.Id,
       TimeZoneSidKey='America/Los_Angeles', UserName='xsfdc@testorg.com');

       System.runAs(u) {
         // The following code runs as user 'u'
       System.debug('Current User: ' + UserInfo.getUserName());
       System.debug('Current Profile: ' + UserInfo.getProfileId());
    
    
       Lead objLead=new Lead();
       objLead.OwnerId=u.ID;
       objLead.LastName='sampath';
       objLead.Company='Testing';
       objLead.Job_Function__c='VP1';
       objLead.Department__c='HR1!';
       objLead.Lead_Source_Category__c='Partner1';
       objLead.Status='Open';
       objLead.Email='sampath.kumar@octaware.com';
       objLead.phone='515222632';
       insert objLead;
       Lead la = [select phone from Lead where id = :objLead.id limit 1];
       System.debug('Original Lx:'+ la.phone);
       System.assert(la.Phone == '515222632');       
       la.phone = '123456789';
       update la;
      
        Lead lb = [select phone from Lead where id = :objLead.id limit 1];
        System.debug('Changed lb:'+ la.phone);
//COMMENT:  then we assert that the field we updated equals the field it was updated from
        System.assert(la.Phone == '123456789');
        }
      
    }
  }