Implementação do nono dígito com o Masked Input Plugin (http://digitalbush.com/projects/masked-input-plugin/).

gistfile1.js

var changePhoneMask = function () {
    var phone = $(this).val().replace(/\D+/g, '');

    if (phone.length == 2) {
        if ($.inArray(phone, [11, 12, 13, 14, 15, 16, 17, 18, 19] > -1)) {
            $(this).unmask().mask('(99) 99999-9999');
        } else {
            $(this).unmask().mask('(99) 9999-9999');
        }
    }

    $(this).val(phone).caret(5, 5).triggerHandler('input'); // Firefox
    $(this).triggerHandler('paste'); // IE
};

$(function () {
    $('#phone').mask('(99) 9999-9999');
    $('#mobile').mask('(99) 9999-9999');
    $('#mobile').on('keyup', changePhoneMask);
});