JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://jquery-joshbush.googlecode.com/files/jquery.maskedinput-1.2.2.js"></script>
<label for="mobil_phone">Мобильный телефон:</label>  
<input type="text" name="mobil_phone" id="mobil_phone" />

<select id="country" name="country">
	<option value="0" selected="selected">Выбор</option>
	<option value="176" >Россия</option>
	<option value="240">Украина</option>
</select>

JavaScript

$('#country').change(function () {
    $("#mobil_phone").unmask();
    
    if ($('#country option:selected').val() == '176') {
       $("#mobil_phone").mask("+7 (999) 999-99-99");
    } else {
        $("#mobil_phone").mask("+38 (999) 999-99-99");
    }
});