Widget area below header

I’m afraid that’s not possible at the moment, but we’ll consider adding hooks to make that possible. You can follow our progress here:
https://github.com/Automattic/jetpack/issues/1555

Thanks for the feedback!

Here is how you can use one of the new hooks:

function rolyestemonio_custom_mobile_header() { ?>
<header id="branding" role="banner">Default Title Site</header>
 <div class="phone-widget"><a href="tel:551666">551-666</a></div>
<?php }
add_action( 'jetpack_mobile_header_after', 'rolyestemonio_custom_mobile_header' );

To output a shortcode, something like this would do the trick:

function rolyestemonio_custom_mobile_header_shortcode() {
echo do_shortcode( '[your-shortcode]' );
}
add_action( 'jetpack_mobile_header_after', 'rolyestemonio_custom_mobile_header_shortcode' );