$args WHERE "own field" = "something"

Hello, I recently started looking on wordpress and trying to figure out how the querys works. It seems like the easiest way to get a post is like below?

$args = array( 'numberposts' => 1, 'category' => 13, 'orderby' => 'rand' );
            $rand_posts = get_posts( $args );
            foreach( $rand_posts as $post ) : ?>
               <h2 style="margin:0px;"><?php echo the_title(); ?></h2>
            <?php
            endforeach; ?>

But if I want to add to $args that I only want to show the posts where my own field for example phone number is filled out, how do I do that?
For printing the row out on the page I use the follow line, but I can’t figure out how to put it into the $args array.

get_post_meta($post->ID, 'phonenumber', true);

Thanks!