I am using the Delta theme so there was phone number field, i didnt add it as custom. What settings should i use for the country list to show up on phone number field? Currently i have the default settings on the plugin

so current settings are:
Custom Field Phone Identifier: empy
Phone Number CSS Selector: input[name="s_phone_mobile"], input[name="sPhone"]

Make sure code is added to this page and that there are no javascript error on your site.

thats how the code looks on the registration page:
<span class="input-box last"><input id="s_phone_mobile" type="text" name="s_phone_mobile" value="" placeholder="+XXX XXX XXX"></span>

There are no js errors (site is kalosex com )

@KaloSex
If you check for "s_phone_mobile" in source code on registration page, there is no related code (javascript) that should be added in footer most probably.
Didn't you remove some piece in footer.php of theme? hook?

on the bottom of user-register.php i see this:

  <script type="text/javascript">
    $(document).ready(function(){
      $('input[name="s_name"]').attr('placeholder', '<?php echo osc_esc_js(__('First name, Last name', 'delta')); ?>').attr('required', true);
      $('input[name="s_email"]').attr('placeholder', '<?php echo osc_esc_js(__('[email protected]', 'delta')); ?>').attr('required', true).prop('type', 'email');
      $('input[name="s_phone_mobile"]').attr('placeholder', '<?php echo osc_esc_js(__('+XXX XXX XXX', 'delta')); ?>');
      $('input[name="s_password"]').attr('placeholder', '<?php echo osc_esc_js(__('YourPass123!', 'delta')); ?>').attr('required', true);
      $('input[name="s_password2"]').attr('placeholder', '<?php echo osc_esc_js(__('YourPass123!', 'delta')); ?>').attr('required', true);
    });
  </script>

and no, i havent modified the footer.php

ok i tried the solution you giving here and worked: https://forums.osclasspoint.com/sms-notification-and-verification-plugin/prefix-not-showing/msg38212/#msg38212
but this is the only way? To add it on all pages?

Ok yes that's good solution.
It will add script to your registration page always.
Originally by plugin it put it there under certain conditions.