If all you are trying to do is show an element on your page when checkbox is ticked\unticked, then you can just do this on the client with a bit of jQuery - no need to have the whole form post back and return script
1) Edit your checkbox control, make sure AutoPostBack is set to false.
2) Add the following script to your page
$(document).ready(function() {
$('#ch_p').click(function() {
if( $(this).is(':checked')) {
$('#phnoe').show()
} else {
$('#phnoe').hide();
}
});
});
This will show\hide based on whether your checkbox is ticked