html button disable knockout

1<p>
2    <input type='checkbox' data-bind="checked: hasCellphone" />
3    I have a cellphone
4</p>
5<p>
6    Your cellphone number:
7    <input type='text' data-bind="value: cellphoneNumber, enable: hasCellphone" />
8</p>
9 
10<script type="text/javascript">
11    var viewModel = {
12        hasCellphone : ko.observable(false),
13        cellphoneNumber: ""
14    };
15</script>
16