Second phone field does not appear

In the Back office , when adding or editing a listing, there is a second phone number field that—once filled in—does not appear anywhere.

If you mean "other contact", it's open field that may or may not be integrated in theme.

It’s a field for a phone number. How do I integrate it into the BETA theme?

field for phone number is as follows
    <?php if(osc_item_contact_phone() != '' && osc_item_show_phone()) { ?>
      <p class="phone bld"><span><?php _e('Phone', 'sigma'); ?>:</span> <?php echo '<a href="tel:' . osc_item_contact_phone(true) . '">' . osc_item_contact_phone(false) . '</a>'; ?></p>
    <?php } ?>

Regarding the phone field, you should re-examine it. Neither registered nor unregistered users can enter information into it, as it does not appear anywhere. It is visible only in the administration (ADMIN) area.