Unidirectional association with WillCascadeOnDelete
===

I have a very simple unidirectional class mappings. 


    public class MyDbContext : DbContext
    {
        public MyDbContext() : base("CodeFirstDatabase")
        {
            
        }

        public DbSet<Contact> Contacts { get; set; }
        public DbSet<PhoneNumber> Number { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
            modelBuilder.Conventions.Remove<IncludeMetadataConvention>();
            modelBuilder.Entity<Contact>()
                .HasMany(c => c.Numbers)
                .WithOptional()
                //.Map(p => p.MapKey("ContactId"))
                .WillCascadeOnDelete(true);
        }
    }

As you can see, Contact has a list of Numbers, but Number doesn't know anything about contact (it doesn't have a property on it's parent).
What I'm trying to do here is to achieve the following:
when number is removed from contact and saved using SaveChanges, I want that number row to be removed from database, but instead, EF sets the foreign key to null and the row remains in database. Any idea what's wrong?

Thanks.