ok, using rays example of the phone number field
Now update it to use:
<?php echo((isset($_POST['callMe'])?$_POST['phoneNumber']:"(222) 222-2222")); ?>
you could update that to compare the call me field to a specific value:
<?php echo(((isset($_POST['callMe']) && $_POST['callMe'] = "yes")?$_POST['phoneNumber']:"(222) 222-2222")); ?>