Nono Dígito do Lado Esquerdo - Aceita 8 ou 9 Dígitos

    $("#telefone")
        .mask("(99) 9999?-99999")
        .focusout(function (event) {
        var target, phone, element;
        target = (event.currentTarget) ? event.currentTarget : event.srcElement;
        phone = target.value.replace(/\D/g, '');
        element = $(target);
        element.unmask();
        if(phone.length > 10) {
            element.mask("(99) 99999-999?9");
        } else {
            element.mask("(99) 9999-9999?9");
        }
    });