"IsChanged" Trigger and its unit test

ContactCompareTrigger.trg

trigger ContactCompareTrigger on Contact (after update) {
//Create an empty Map of Contacts for eventual updates
    Map<ID, Contact> consToUpdate = new Map<Id, Contact>();

    //This is a very common pattern for iterating over the list of records
    //We use the size() method for Lists to handle each Contact in Trigger.new 
    
    for(Integer i = 0 ; i < trigger.new.size() ; i++){
        Contact old = trigger.old[i];
        Contact nw = trigger.new[i];//We cannot use "new" as our variable name, it's a reserved word.

        if(old.Phone != nw.Phone){
            Contact c = new Contact(Id = nw.Id, Description  = 'Phone Changed');
            consToUpdate.put(nw.Id, c);
        }
    }
    
    //BEST PRACTICE: Use size() again to check that you have records to be updated first
    if(consToUpdate.size() > 0){
       update consToUpdate.values();
    }
    
}

ContactCompareTriggerTest.cls

@isTest
public class ContactCompareTriggerTest {
    public static testMethod void testMany(){
        //Must create a test Account for the test Contacts!
        Account a = new Account(Name = 'Test Account');
        insert a;
        
        //Use a for-loop to populate a List of test Contacts
        List<Contact> insertContacts = new List<Contact>();
        for(Integer i = 0; i < 2 ; i++){
            Contact c = new Contact(LastName = 'name' + i,//Use i to make each name unique
                                   AccountId = a.Id);
            insertContacts.add(c);
        }
        insert insertContacts;
        
        //Since our trigger is on AfterUpdate, we must do an update
        List<Contact> updateContacts = new List<Contact>();
        for(Contact c : insertContacts){
            c.Phone = '212-555-0123';
            updateContacts.add(c);
        }
        update updateContacts;
        
        for(Contact c : [Select Description from Contact]){
            System.assertEquals('Phone Changed', c.Description);
        }        
        
        
    }

}