Detect with PHP if its a mobile agent: http://www.000webhost.com/forum/scripts-code-snippets/27404-php-extremely-simple-way-check-if-mobile-browser.html
<?php
if(strstr(strtolower($_SERVER['HTTP_USER_AGENT']), 'mobile') || strstr(strtolower($_SERVER['HTTP_USER_AGENT']), 'android')) {
echo '<a href="tel:123456">';
}else{
echo 'You are not in a mobile';
}
?>