Misleading error message for question of type phone number

[Patta]

If a question of type phone number is active for attendee data and if the user inserts his phone number with country code, he won't really be able to figure out the error, because the sample number in the error message is in the format he entered it. It would be better to also describe in the error message that the country code must be selected separately.

Example:

[raphaelm]

This is technically a bug in https://github.com/stefanfoulis/django-phonenumber-field

Let's see if we can do something about it