<input type="tel" id="select__flags_main" class="headerform__tel input__text input__gray input__hover" name="flags" placeholder="Телефон *">

var mainform = document.querySelectorAll(".headerform__tel");
 for (var i = 0; i < mainform.length; i++) {
  intlTelInput(mainform[i], {
   defaultCountry: "ru",
   autoPlaceholder: true,
   onlyCountries: ['ru', 'by', 'ua', 'kz'],
  });
}

.setNumber("+447733123456")