I have created a trigger which simply fails to update a field and I can't see why.

The tirgger looks at a new task coming in and compares the phone number on the task (t.Five9__Five9DNIS__c) with a phone number on the contact.  If they don't match.  Then it sets a custom field on the task.

I have have set up a test.   When I run it I use debug statements in the trigger to verify:  

* the trigger correctly identifies the mismatch

* the value of the field is changed.

However, the assertEquals (the final line in the test) fails!

What gives?

The Trigger:

trigger CheckPhoneMatch on Task (before insert, before update) {

    list<id> contactIds = new list<id>();
    for (Task t : trigger.new)
    {
        contactIds.add(t.whoid);
    }
    
    map<id,contact> contactMap = new map<id,contact>([Select Id, FirstName, LastName, DonorId__c, Phone from contact Where Id in :contactIds ]);
    // list<task> updatedTasks = new list<task>();
    
    for (Task t : trigger.new)
    {
        contact c = contactMap.get(t.whoid);
        
        System.Debug('/n/n*******       Phone ' + c.Phone + ' DNIS: ' + t.Five9__Five9DNIS__c + '       ********');
        
        
        if (c != null)
        {                        
            if (t.Five9__Five9DNIS__c != c.Phone)
            {
                System.Debug('DNIS DOES NOT MATCH');
                t.MismatchPhone__c = true;
                System.Debug('VAL IS: ' + t.MismatchPhone__c);                
            }                                    
        }
    }
}

The Test (only the final block of code is relevant):

@istest
private class TriggerTests {

   static testMethod void contactBeforeUpdate()
   {
     contact c = new contact();
     c.lastname = 'Smith';
     insert c;
     
     c.address1__c = '123 Main Street';
     c.email = 'test@company.com';
     c.Phone = '1234567890';
     update c;

        
     user u2 = [select id from user where isactive = true limit 1];
   
     task t = new task();
     t.ownerid = u2.id;
     t.whoid = c.id;
     t.subject = 'test';
     t.CallDisposition = 'Yes - Gift Info Acquired';
     t.Five9__Five9DNIS__c = '1234567890';
     
     insert t; 
          
     System.assertEquals(false, t.MismatchPhone__c);

     <OMITTING IRRELEVANT TESTS>               
     
     task t3 = new task();
     t3.ownerid = u2.id;
     t3.whoid = c.id;
     t3.subject = 'test2';
     t3.CallDisposition = 'Callback';   
     t3.Five9__Five9DNIS__c = '0987654321';
 
     insert t3;
     
     // THIS LINE FAILS!!!!!
     System.assertEquals(true, t3.MismatchPhone__c);
 
   }

}