is_mobile() in functions.php

Hi,
is there a way to determine if we are on a mobile device in functions.php() ?

Background is this:
I added a function to display a phone number in a specific way, like so:

function add_stuff_after_main_menu() {
	$is_mobile = True;
	if ( is_front_page() || !is_front_page() && $is_mobile ) {
		echo '<div style="float:right;">';
		echo '<div style="margin-top: -25px;"><a class="phoneicon phonelink" href="tel:+4912345">+49 12345</a></div>';
		echo '</div>';
	}
}
add_action('ava_after_main_menu', 'add_stuff_after_main_menu'); 

This works as expected, but for my variable $is_mobile I need a function to get the real functionality.

Is there a way where I can get the needed information?

Thanks in advance,
Michael