The simplest option may be to change the input elements to type "number." That could be achieved by adding this script to the CBC:
<script>
$(document).ready(function(){
$('#[% QuestionName() %]_div input[type="tel"]').attr('type', 'number');
})
</script>
How the input responds to this change is browser-dependent, though, so you'd need to test to be confident about device agnosticism. If this approach is insufficient, adding this script will add the two buttons you describe:
<script>
$(document).ready(function(){
// Settings
var upLabel = '+';
var downLabel = '-';
// Run
$('#[% QuestionName() %]_div input[type="tel"]').each(function(){
var input = this;
var downButton = $('<button type="button">' + downLabel + '</button>');
$(downButton).click(function(){
var value = Number($(input).val()) || 0;
$(input).val(value - 1);
});
$(input).after(downButton);
var upButton = $('<button type="button">' + upLabel + '</button>');
$(upButton).click(function(){
var value = Number($(input).val()) || 0;
$(input).val(value + 1);
});
$(input).after(upButton);
});
})
</script>
You may want to also apply some CSS to define how these buttons look, but that code would be dependent on your visual requirements.