Phone number issue

You’re overriding the phone number field, and adds t.ex. +47 automatically into it for Norway.

This causes a lot of customers to skip the field as it’s already filled, and have no further validation on the phone number length / formatting – so they can checkout without filling it.

Better remove your override, or add a proper phone number validation, else it’s worse as is than you not getting the country prefix (something carriers should be able to handle elsewhere anyway ;)).

Just an example for a simple validation, needs something more solid for all the world:

add_action('woocommerce_checkout_process', 'wc_validate_nordic_phone');

function wc_validate_nordic_phone() {

    if (empty($_POST['billing_phone'])) {
        return;
    }

    $raw = trim(wp_unslash($_POST['billing_phone']));

    // Normalize: keep leading + if present, remove everything but digits
    $has_plus = (strpos($raw, '+') === 0);
    $digits   = preg_replace('/\D/', '', $raw);

    // Convert 00 prefix to "international" (same treatment as +)
    $is_international = false;
    if ($has_plus) {
        $is_international = true;
        // the digits are already without +
    } elseif (strpos($digits, '00') === 0) {
        $is_international = true;
        $digits = substr($digits, 2); // remove 00
    }

	// Country rules: national length (without country code) for mobile
	// [country code => [min, max]]
    $rules = [
        '47'  => [8, 8],   // Norway
        '46'  => [9, 9],   // Sweden (mobile, without leading 0)
        '45'  => [8, 8],   // Denmark
        '358' => [9, 10],  // Finland (mobile varies)
        '354' => [7, 7],   // Iceland
        '298' => [6, 6],   // Faroe Islands
        '299' => [6, 6],   // Greenland
    ];

    $country_code = null;
    $national     = $digits;

    if ($is_international) {
        // Find matching country code (longest first to hit 358/354/298/299 before 3/5/2/9)
        $codes = array_keys($rules);
        usort($codes, function ($a, $b) { return strlen($b) - strlen($a); });

        foreach ($codes as $code) {
            if (strpos($digits, $code) === 0) {
                $country_code = $code;
                $national     = substr($digits, strlen($code));
                break;
            }
        }

        if ($country_code === null) {
            wc_add_notice(__('Invalid country code. Use +47, +46, +45, +358, +354, +298 eller +299.', 'posten-bring-checkout'), 'error');
            return;
        }
    } else {
        // National format – assume the store's country, fall back to Norway
        $store_country = WC()->customer ? WC()->customer->get_billing_country() : 'NO';
        $country_map = [
            'NO' => '47', 'SE' => '46', 'DK' => '45',
            'FI' => '358', 'IS' => '354', 'FO' => '298', 'GL' => '299',
        ];
        $country_code = $country_map[$store_country] ?? '47';

        // Remove any leading 0 (common in national notation for SE/FI)
        $national = ltrim($national, '0');
    }

    [$min, $max] = $rules[$country_code];
    $len = strlen($national);

    if ($len < $min || $len > $max) {
        wc_add_notice(
            sprintf(
                /* translators: 1: country code, 2: min, 3: max */
                __('Invalid mobile number for +%1$s. Expected %2$d–%3$d digits after the country code.', 'posten-bring-checkout'),
                $country_code, $min, $max
            ),
            'error'
        );
    }
}