Improve accessibility of validation error

Thanks for your report.

This module uses the default validation handling (e.g. https://git.drupalcode.org/project/telephone_advanced/-/blob/1.x/src/Plu...), so this message is themed by your theme. I suggest you open an issue in that project, since I cannot fix it here.

This might actually be partly a core issue where the module is using the $form_state->setError() function.

$form_state->setError($element, t('The phone number %phone is not valid.', ['%phone' => $value]));