Automatically dial a phone number

On mobile devices, there are protocol handlers to launch the phone. Depending on the security, some will dial it, or others will bring on the phone application with the number already there.

<a href="tel:+15555555555">Call me at +1 (555) 555-5555</a>

I've used this in the past for mobile apps:

<button id="call">Call</button>

<script>
    $("#call").on('click', function() {
        var link = "tel:18003334444";
        window.location.href = link;
    });
</script>

This, of course, uses jQuery.