I am trying to incorporate into my project a masked input plug-in (URL: http://digitalbush.com/projects/masked-input-plugin/)

When I add this .js

<script src="Scripts/jquery-1.7.1.js" type="text/javascript"></script>
<script src="Scripts/jquery.maskedinput-1.2.2.min.js" type="text/javascript"></script>

to a separate .aspx file in my project, the mask() function works very well.

But when I add it to a page that uses Master page, I get error:

0x800a01b6 - Microsoft JScript runtime error: Object doesn't support property or method 'mask'

The test code is identical in separate test page and the page that uses master page:

 <script type="text/javascript">
        $(document).ready(function () {
             $("#txtPhoneNumber").mask("(999) 9999-9999");
        })
    </script>

The error (above) happens on the line .mask() above.

How would you recommend to debug this situation? TIA