How to turn each phone number within the $content to a telephone link?

You can achieve it by using following custom code.

function accessnumber( $content ) {
    $content = preg_replace( '#(<div[^>]*class=\"zebra[^>]*\">)(.*?)(</div>)#is', '$1<a href="https://wordpress.stackexchange.com/questions/266577/tel:">$2</a>$3', $content );
    return $content;
}
add_filter( 'the_content', 'accessnumber', 60 );