How to fix Nextcloud Can not validate phone numbers without `default_phone_region` being set in the config file

Problem:

When running NextcloudĀ occ maintenance:repair you see an error message like

ERROR: Can not validate phone numbers without `default_phone_region` being set in the config file

Solution:

Add the config option using

php occ config:system:set default_phone_region --type string --value="DE"

or, when using docker-compose:

docker-compose exec -u www-data nextcloud php occ config:system:set default_phone_region --type string --value="DE"

and runĀ maintenance:repair again.