Edit in JSFiddle

// UNCOMMENT THIS AT SHOP
//require(['jquery', '$ready!'], function ($) {


// Puhelinnumeron validointi
if($('div#BasketAddressFormContainer, form#registrationForm').length){
    $('<p class="puhelin-huomio hidden">Tarkista puhelinnumerosi muoto!</p>').insertAfter('input[name=Phone]');
    
    var $regexname=/^(\+358|0)[\d| ]{0,10}$/;
    $('input[name=Phone]').on('keypress keydown keyup',function(){
             if (!$(this).val().match($regexname)) {
              // there is a mismatch, hence show the error message
                 $('.puhelin-huomio').removeClass('hidden');
             }
           else{
                // else, do not display message
                $('.puhelin-huomio').addClass('hidden');
               }
         });
 }

// UNCOMMENT THIS AT SHOP
//});
div#BasketAddressFormContainer p.hidden, form#registrationForm p.hidden {
    display: none;
}
div#BasketAddressFormContainer p.puhelin-huomio, form#registrationForm p.puhelin-huomio {
    color: red;
}