html button disable knockout

<p>
    <input type='checkbox' data-bind="checked: hasCellphone" />
    I have a cellphone
</p>
<p>
    Your cellphone number:
    <input type='text' data-bind="value: cellphoneNumber, enable: hasCellphone" />
</p>
 
<script type="text/javascript">
    var viewModel = {
        hasCellphone : ko.observable(false),
        cellphoneNumber: ""
    };
</script>