How do I echo all users’ display_name and their meta_value who have a certain meta_key?

I want to echo a list of all users who have that field; in other
words, a list of each user’s name, phone number, and the quantity.

Try this, which is based on your 1st snippet:

( and this is all you need; no need for the $wpdb->get_col() snippet )

$user_query = new WP_User_Query( array( 'meta_key' => 'sms_subscriber' ) );
$users = $user_query->get_results();
if (!empty($users)) {
    echo '<h3>SMS Subscribers (' . $user_query->get_total() . ')</h3>';

    echo '<ul>';
    foreach ($users as $user){
        // this one uses the magic __get() method in WP_User, i.e. $user-><meta key>
//      echo ' <li>' . $user->display_name . " $user->sms_subscriber" . '</li>';

        $sms_subscriber = get_user_meta( $user->ID, 'sms_subscriber', true );
        echo ' <li>' . $user->display_name . " $sms_subscriber" . '</li>';
    }
    echo '</ul>';
}

Things to note:

• You can use get_user_meta() to retrieve the value of a user meta.

  Alternatively, WP_User has a magic __get() method which you can use to get a meta value without having to use the above function, but only to get a single meta value (regardless if that value is a string/text, number, array, etc.). So for example, you could use $user->sms_subscriber like you could see in the code I commented out above.

• To get the “quantity” or sum as you said it, which is the total number of users that matched your query arguments, you can use WP_User_Query::get_total().

  If you just wanted to count the number of items in $users (i.e. the 1st page of the query’s results — the results can be paginated using the number argument), then you’d use count( $users ) in place of the $user_query->get_total().