$(document).ready(function() {
      $('#user-list').select2({
        ajax: {
          url: "<?php echo base_url() . 'User_basic/fetch_mobile_numbers'; ?>",
          dataType: 'json',
          type: "post",
          delay: 250,
          data: function (params) {
           return {
             searchTerm: params.term // search term
           };
          },
          processResults: function (response) {
           console.log(response);
           return {
            results: response
          };
        },
       }
    });
 });

<select name="phone_number" id="user-list" placeholder="Enter Number" class="form-control" multiple="multiple" required>
   <option value="" id="numbers-list"></option>
</select>