<input type="checkbox" id="chk1" name="chk1" value="100">Voicemail<br />
<input type="checkbox" id="chk2" name="chk2" value="50">VM Support

$('#chk1, #chk2').on('click', function(){
    var checked = $(this).is(':checked');
    $('#chk1, #chk2').attr('checked', checked);
});

<input type="checkbox" id="chk1" name="chk1" value="100">
<label for="chk1">Voicemail and VM Support</label>

$('#chk1, #chk2').click(function(){
     checked = this.checked;
     $('#chk1, #chk2').each(function(){
         $(this).attr('checked', checked);
     });
});

$('#chk1 , #chk2').on('click', function(){

    $('#chk1 , #chk2').attr('checked', $(this).is(':checked'))
});