Require one of two fields be filled out

This code makes it so that a user must fill out one of two possible fields.

// Grab variables.
$email = $form_values['submitted_tree']['email_address'];
$phone = $form_values['submitted_tree']['phone_number'];

// Check for values.
if ($email == "" && $phone == "") {
  form_set_error('submitted][email_address', t('E-mail Addresses or Phone Number must be given.'));
  form_set_error('submitted][phone_number', '');
}

Requires that an email or phone number is given.