Our numeric question may already suit your needs for this task. If you need this behavior for an open-end, you'll need to use JavaScript to make some changes to the HTML, but there are a few ways to do this that might result in some keyboard differences on mobile devices. One simple way would be to simply change type=text to type=tel:
<script>
$(document).ready(function(){
$('#[% QuestionName() %]').attr('type', 'tel');
})
</script>