Form Validation: Custom Callback Rule Problem...

You mention that this is the 2nd page of your form. I guess the form_validation rule is applied after you post the 1st page.
Please, share the logic in your controller that handles the form pages and $_POST result.
And please, don't paste your php code as plain text in the forum editor. There's a button for php code in the button bar. If you use that, you make the code much better readable for everyone.